The following test in Transforms/InstCombine/vector-xor.ll shows an incorrect transformation. I reproduce it here with a smaller vector size & bitwidth to make it easier to understand.
The issue is that "undef >> 4" is "ssss.sxyz" (s = sign bit), while "undef u>> 4" is "0000.xyzw", so it can produce the value "0000.1000", while ashr cannot. See details below:


define <2 x i8> @test_v4i32_not_ashr_negative_const_undef(<2 x i8> %a0) {
  %1 = ashr <2 x i8> { 251, undef }, %a0
  %2 = xor <2 x i8> { 255, 255 }, %1
  ret <2 x i8> %2
}
=>
define <2 x i8> @test_v4i32_not_ashr_negative_const_undef(<2 x i8> %a0) {
  %1 = lshr <2 x i8> { 4, undef }, %a0
  ret <2 x i8> %1
}
Transformation doesn't verify!
ERROR: Value mismatch

Example:
<2 x i8> %a0 = < #x00 (0), #x04 (4) >

Source:
<2 x i8> %1 = < #xfb (251, -5), #x00 (0)        [based on undef value] >
<2 x i8> %2 = < #x04 (4), #xff (255, -1) >

Target:
<2 x i8> %1 = < #x04 (4), #x08 (8) >
Source value: < #x04 (4), #xff (255, -1) >
Target value: < #x04 (4), #x08 (8) >