I observe the same assertion when compiling a simple .c test with msan:

typedef char int8_t;
int8_t a;
b;
c;
int8_t
fn1 (p1) {
return a < 0 || p1 || 0 > 1 >> p1 ? a : a << p1;
}
fn2 () {
int8_t d;
c = fn1 (b << 13);
d = c < 0 ? : 0;
fn3 (d);
}

% clang -c -O3 -g -fsanitize=memory j.c

llvm/include/llvm/Support/Casting.h:97: static bool llvm::isa_impl_cl<llvm::ConstantSDNode, const llvm::SDNode *>::doit(const From *) [To = llvm::ConstantSDNode, From = const llvm::SDNode *]: Assertion `Val && "isa<> used on a null pointer"' failed.
11 clang-3.3 0x0000000000b62eed llvm::SelectionDAG::Combine(llvm::CombineLevel, llvm::AliasAnalysis&, llvm::CodeGenOpt::Level) + 733
12 clang-3.3 0x0000000000c1f3a7 llvm::SelectionDAGISel::CodeGenAndEmitDAG() + 871
13 clang-3.3 0x0000000000c1e7d9 llvm::SelectionDAGISel::SelectAllBasicBlocks(llvm::Function const&) + 6265
14 clang-3.3 0x0000000000c1c2a6 llvm::SelectionDAGISel::runOnMachineFunction(llvm::MachineFunction&) + 1302
15 clang-3.3 0x0000000000d6d30d llvm::MachineFunctionPass::runOnFunction(llvm::Function&) + 125

I observe the same assertion when compiling a simple .c test with msan:
This means there's another wrong usage of isa<> function, but this is an absolutely unrelated issue.

Fixed in r195016.