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

[RiverDave]

The _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 on CIR, this optimization makes sense; however, if we want to preserve parity with OG, I believe this patch matches that behaviour.

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.