I managed to pinpoint further. LNT fails with the following command line:

mysandbox/bin/lnt runtest nt --sandbox /tmp/bar --cc /home/grosser/Projects/polly/build/bin/clang --cxx /home/grosser/Projects/polly/build/bin/clang++ --test-suite test-suite/ -j4 --only-test MultiSource/Applications/JM/lencod --mllvm=-polly --multisample=1 --mllvm=-polly-process-unprofitable --mllvm=-polly-invariant-load-hoisting --mllvm=-debug-only=polly-scops --mllvm=-polly-only-func=dct_luma_16x16 --mllvm=-polly-only-region=for.body32

The scop in question has the following invariant loads:

Function: dct_luma_16x16
Region: %for.body32---%for.inc64
Max Loop Depth:  1
Invariant Accesses: {
        ReadAccess :=	[Reduction Type: NONE] [Scalar: 0]
            [p_0, p_1, p_2, p_3, new_intra_mode, p_5] -> { Stmt_for_body32[i0] -> MemRef_imgY_org[0] };
        Execution Context: [p_0, p_1, p_2, p_3, new_intra_mode, p_5] -> {  :  }
        ReadAccess :=	[Reduction Type: NONE] [Scalar: 0]
            [p_0, p_1, p_2, p_3, new_intra_mode, p_5] -> { Stmt_for_body32[i0] -> MemRef_27[o0] : 96 <= o0 <= 97 };
        Execution Context: [p_0, p_1, p_2, p_3, new_intra_mode, p_5] -> {  : 1 = 0 }
        ReadAccess :=	[Reduction Type: NONE] [Scalar: 0]
            [p_0, p_1, p_2, p_3, new_intra_mode, p_5] -> { Stmt_for_body32[i0] -> MemRef_32[p_0 + p_1] };
        Execution Context: [p_0, p_1, p_2, p_3, new_intra_mode, p_5] -> {  : 1 = 0 }
}

Interestingly, the scop itself just consists of a single basic block, which is always executed at least once. So the only situation under which the execution context could be empty without us loading garbage from memory is a situation where we fall back to the original code. However, the run-time check only contains (besides the alias checks) a couple of overflow conditions that are unlikely to trigger:

0 == (p_0 + p_1 <= -2147483649 || p_0 + p_1 >= 2147483648 || p_2 >= 2147483633 || p_0 <= -1)

#29354 showed also an empty execution context, such that the fix we applied at this point (https://llvm.org/svn/llvm-project/polly/trunk@279047) could have caused a regression here. However, I confirmed this is not the case, as the empty execution context remains even with r279047 reverted.

https://llvm.org/svn/llvm-project/polly/trunk@279047

With r286781 we see at the following compile-time crash:

#​1 0x00007fd8d41d2626 SignalHandler(int) (/home/grosser/.data-unbackupped/polly-build/bin/../lib/libLLVMSupport.so.40+0xf1626)
#​2 0x00007fd8d76d4670 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x11670)
#​3 0x00007fd8d1e7b7ef gsignal /build/glibc-DfDqKW/glibc-2.24/signal/../sysdeps/unix/sysv/linux/raise.c:58:0
#​4 0x00007fd8d1e7d3ea abort /build/glibc-DfDqKW/glibc-2.24/stdlib/abort.c:91:0
#​5 0x00007fd8d1e73bb7 __assert_fail_base /build/glibc-DfDqKW/glibc-2.24/assert/assert.c:92:0
#​6 0x00007fd8d1e73c62 (/lib/x86_64-linux-gnu/libc.so.6+0x2dc62)
#​7 0x00007fd8d5f53a0b polly::Scop::getNonHoistableCtx(polly::MemoryAccess*, isl_union_map*) (/home/grosser/.data-unbackupped/polly-build/bin/../lib/libPolly.so+0x86a0b)
#​8 0x00007fd8d5f56ef3 polly::Scop::hoistInvariantLoads() (/home/grosser/.data-unbackupped/polly-build/bin/../lib/libPolly.so+0x89ef3)
#​9 0x00007fd8d5f55e5f polly::Scop::init(llvm::AAResults&, llvm::AssumptionCache&, llvm::DominatorTree&, llvm::LoopInfo&) (/home/grosser/.data-unbackupped/polly-build/bin/../lib/libPolly.so+0x88e5f)
#​10 0x00007fd8d5f6e0fb polly::ScopBuilder::buildScop(llvm::Region&, llvm::AssumptionCache&) (/home/grosser/.data-unbackupped/polly-build/bin/../lib/libPolly.so+0xa10fb)
#​11 0x00007fd8d5f6e3b0 polly::ScopBuilder::ScopBuilder(llvm::Region*, llvm::AssumptionCache&, llvm::AAResults&, llvm::DataLayout const&, llvm::DominatorTree&, llvm::LoopInfo&, polly::ScopDetection&, llvm::ScalarEvolution&) (/home/grosser/.data-unbackupped/polly-build/bin/../lib/libPolly.so+0xa13b0)
#​12 0x00007fd8d5f5ccb4 polly::ScopInfoRegionPass::runOnRegion(llvm::Region*, llvm::RGPassManager&) (/home/grosser/.data-unbackupped/polly-build/bin/../lib/libPolly.so+0x8fcb4)
#​13 0x00007fd8d5af944f llvm::RGPassManager::runOnFunction(llvm::Function&) (/home/grosser/.data-unbackupped/polly-build/bin/../lib/libLLVMAnalysis.so.40+0x1d144f)