[InstCombine] enhance icmp with sub folds
There were 2 related but over-specified folds for:
C1 - X == C
One allowed multi-use but was limited to equal constants.
The other allowed different constants but disallowed multi-use.
This combines the 2 folds into a more general match.
The test diffs show the multi-use cases that were falling
through the cracks.
https://alive2.llvm.org/ce/z/4_hEt2
define i1 @src(i8 %x, i8 %subC, i8 %C) {
%s = sub i8 %subC, %x
%r = icmp eq i8 %s, %C
ret i1 %r
}
define i1 @tgt(i8 %x, i8 %subC, i8 %C) {
%newC = sub i8 %subC, %C
%isneg = icmp eq i8 %x, %newC
ret i1 %isneg
}