[InstCombine] Extend canonicalizeClampLike to handle truncated inputs

This extends the canonicalizeClampLike function to allow cases where the
input is truncated, but still matching on the types of the ICmps. For
example
  %t = trunc i32 %X to i8
  %a = add i32 %X, 128
  %cmp = icmp ult i32 %a, 256
  %c = icmp sgt i32 %X, -1
  %f = select i1 %c, i8 High, i8 Low
  %r = select i1 %cmp, i8 %t, i8 %f
becomes
  %c1 = icmp slt i32 %X, -128
  %c2 = icmp sge i32 %X, 128
  %s1 = select i1 %c1, i32 sext(Low), i32 %X
  %s2 = select i1 %c2, i32 sext(High), i32 %s1
  %t = trunc i32 %s2 to i8
https://alive2.llvm.org/ce/z/vPzfxH

We limit the transform to constant High and Low values, where we know
the sext are free.

Differential Revision: https://reviews.llvm.org/D108049