Extended Description

In case of signed integer wrap of the backedge-taken count from the SCEV, Polly generate empty domains.

See attached test case:
$ opt -load LLVMPolly.so -scalar-evolution -polly-scops -analyze test.ll
Printing analysis 'Scalar Evolution Analysis' for function 'test':
Classifying expressions for: @​test
%i = phi i7 [ 0, %preheader ], [ %i.1, %body.incr ]
--> {0,+,1}<%header> Exits: -1
%tmp = zext i7 %i to i64
--> {0,+,1}<%header> Exits: 127
%A.addr = getelementptr [128 x i32]* @​A, i64 0, i64 %tmp
--> {@A,+,4}<%header> Exits: (508 + @​A)
%A.load = load i32* %A.addr, align 4
--> %A.load Exits: <>
%A.inc = zext i7 %i to i32
--> {0,+,1}<%header> Exits: 127
%A.val = add nsw i32 %A.load, %A.inc
--> ({0,+,1}<%header> + %A.load) Exits: <>
%i.1 = add i7 %i, 1
--> {1,+,1}<%header> Exits: 0
Determining loop execution counts for: @​test
Loop %header: backedge-taken count is -1
Loop %header: max backedge-taken count is -1
Printing analysis 'Polly - Create polyhedral description of Scops' for region: 'header => exit' in function 'test':
Context:
{ : }
Statements {
Stmt_body_load
Domain :=
{ Stmt_body_load[i0] : 1 = 0 };
Scattering :=
{ Stmt_body_load[i0] -> scattering[0, i0, 0] };
ReadAccess :=
{ Stmt_body_load[i0] -> MemRef_A[i0] };
MustWriteAccess :=
{ Stmt_body_load[i0] -> MemRef_A_load_s2a[0] };
Stmt_body_store
Domain :=
{ Stmt_body_store[i0] : 1 = 0 };
Scattering :=
{ Stmt_body_store[i0] -> scattering[0, i0, 1] };
ReadAccess :=
{ Stmt_body_store[i0] -> MemRef_A_load_s2a[0] };
MustWriteAccess :=
{ Stmt_body_store[i0] -> MemRef_A[i0] };
}

Reason:
SCEV of backedge-taken count is parsed wrongly assuming signed value. (See SCEVAffinator::visitConstant)
C/C++ forbid signed wrap but LLVM IR does not.

Good fix:
Better parsing of SCEV taking sign into account.

Poor fix:
Forbid backedge-taken count to have sign-wrap, at ScopDetection. (Reject if count is negative)