failed to fold shift amount offset into shift of a constant, part 2

[rotateright]

Forking this off from #54890 - that handled a shift-left variant, but there's a similar shift-right fold that could be added:
https://alive2.llvm.org/ce/z/qzh4oK

define i32 @src(i32 %x, i32 %c, i32 %addc) {
  %a = add i32 %x, -1
  %r = ashr exact i32 -2, %a
  ret i32 %r
}

define i32 @tgt(i32 %x, i32 %c, i32 %addc) {
  %r = ashr i32 -4, %x
  ret i32 %r
}

This should be generalized for other constant values, and it should also work in the more common case of a positive shift constant with "lshr".

[llvmbot]

@llvm/issue-subscribers-good-first-issue