[InstSimplify] allow undefs in icmp with vector constant folds

This is the main icmp simplification shortcoming seen in D84655.

Alive2 agrees that the basic examples are correct at least:

define <2 x i1> @src(<2 x i8> %x) {

%0:

%r = icmp sle <2 x i8> { undef, 128 }, %x ret <2 x i1> %r

}

## >

define <2 x i1> @tgt(<2 x i8> %x) {

%0:

ret <2 x i1> { 1, 1 }

}

Transformation seems to be correct!

define <2 x i1> @src(<2 x i32> %X) {

%0:

%A = or <2 x i32> %X, { 63, 63 } %B = icmp ult <2 x i32> %A, { undef, 50 } ret <2 x i1> %B

}

## >

define <2 x i1> @tgt(<2 x i32> %X) {

%0:

ret <2 x i1> { 0, 0 }

}

Transformation seems to be correct!

https://alive2.llvm.org/ce/z/omt2ee

https://alive2.llvm.org/ce/z/GW4nP_

Differential Revision: https://reviews.llvm.org/D84762