[InstCombine] fold abs of select with negated op (PR39474)

[InstCombine] fold abs of select with negated op (PR39474)

Similar to the existing transform - peek through a select
to match a value and its negation.

https://alive2.llvm.org/ce/z/MXi5KG

  define i8 @src(i1 %b, i8 %x) {
  %0:
    %neg = sub i8 0, %x
    %sel = select i1 %b, i8 %x, i8 %neg
    %abs = abs i8 %sel, 1
    ret i8 %abs
  }
  =>
  define i8 @tgt(i1 %b, i8 %x) {
  %0:
    %abs = abs i8 %x, 1
    ret i8 %abs
  }
  Transformation seems to be correct!