[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
  }