optimize icmp-ugt-ashr

This diff optimizes the sequence icmp-ugt(ashr,C_1) C_2. InstCombine
already implements this optimization for sgt, and this patch adds
support ugt. This patch adds the check for UGT.

@craig.topper came up with the idea and proof:

  define i1 @src(i8 %x, i8 %y, i8 %c) {
    %cp1 = add i8 %c, 1
    %i = shl i8 %cp1, %y
    %i.2 = ashr i8 %i, %y
    %cmp = icmp eq i8 %cp1, %i.2
    ;Assume: C + 1 == (((C + 1) << y) >> y)
    call void @llvm.assume(i1 %cmp)

    ; uncomment for the sgt case
    %j = shl i8 %cp1, %y
    %j.2 = sub i8 %j, 1
    %cmp2 = icmp ne i8 %j.2, 127
    ;Assume (((c + 1 ) << y) - 1) != 127
    call void @llvm.assume(i1 %cmp2)

    %s = ashr i8 %x, %y
    %r = icmp sgt i8 %s, %c
    ret i1 %r
  }

  define i1 @tgt(i8 %x, i8 %y, i8 %c) {
    %cp1 = add i8 %c, 1
    %j = shl i8 %cp1, %y
    %j.2 = sub i8 %j, 1

    %r = icmp sgt i8 %x, %j.2
    ret i1 %r
  }

  declare void @llvm.assume(i1)

  This change is related to the optimizations in D117252.

  Differential Revision: https://reviews.llvm.org/D117365