[CIR][Lowering] Fix Vector Comparison Lowering with -fno-signed-char/unsigned operand

[RiverDave]

While working on _mm_movepi8_mask, intrinsic (and similar sign-bit checking intrinsics containing 8-bit integers) was being optimized away when using -fno-signed-char. Effectively replacing a cmp expression for 0.

Since unsigned values can never be less than zero, the CIR lowering was directly generating a constant 0 (I suppose we fold a vec filled with 0's to our target, which is a scalar mask, which in turn is 0) instead of the intended comparison operation, completely eliminating the icmp instruction.

See when passing as arg -fno-signed-char (no cmp generated):

OG:

define dso_local i16 @test_mm_movepi8_mask(<2 x i64> %0) #0 {
...
  %8 = bitcast <2 x i64> %7 to <16 x i8>
  %9 = icmp slt <16 x i8> %8, zeroinitializer
  store i16 %10, ptr %3, align 2
  ...
  ret i16 %12
}

CIR:

define dso_local i16 @test_mm_movepi8_mask(<2 x i64> %0) #0 {
...
  %8 = bitcast <2 x i64> %7 to <16 x i8>
  store i16 0, ptr %3, align 2 // I believe our vector cmp is folded here?
...
  ret i16 %10
}

Since integer signedness is something we can track, the behaviour CIR is enforcing makes sense; however, if we want to preserve parity with OG, I believe this patch will match that. I can close this PR if that's not applicable to this case.

Added special case detection for sign-bit extraction patterns (lt comparison with cir::ZeroAttr) to force signed comparison regardless of the element type's signedness. This preserves the semantic intent of checking sign bits rather than performing mathematical unsigned comparisons.

[bcardosolopes]

We just went through a rebase, this PR needs to be updated.

[RiverDave]

We just went through a rebase, this PR needs to be updated.

Updated this along with my other currently open PR's

[bcardosolopes]

clang/test/CIR/Lowering/vec-cmp.cir seems to be failing!

[RiverDave]

clang/test/CIR/Lowering/vec-cmp.cir seems to be failing!

My apologies. Tests seems to be fixed now. I'm still waiting for your follow up on the discussion above in order to push forward this PR if required. Thanks 😃