This transformation exposed in Transforms/InstCombine/icmp-vec.ll is incorrect. The issue is that "INT_MIN <= undef" is not the same as "INT_MIN < undef".

define <2 x i1> @PR27756_2(<2 x i8> %a) {
  %cmp = icmp sle <2 x i8> %a, { undef, 0 }
  ret <2 x i1> %cmp
}
=>
define <2 x i1> @PR27756_2(<2 x i8> %a) {
  %cmp = icmp slt <2 x i8> %a, { undef, 1 }
  ret <2 x i1> %cmp
}
Transformation doesn't verify!
ERROR: Value mismatch

Example:
<2 x i8> %a = < #x80 (128, -128), #x80 (128, -128) >

Source:
<2 x i1> %cmp = < #x1 (1), #x1 (1) >

Target:
<2 x i1> %cmp = < #x0 (0), #x1 (1) >
Source value: < #x1 (1), #x1 (1) >
Target value: < #x0 (0), #x1 (1) >