Change CMP with imm to large imm and vice versa to expose CSE opportunities with SUB

[vladimirradosavljevic]

#651 adds support for adjusting CMP instructions to expose CSE opportunities with SUB. That PR doesn't add support where adjusting CMP would require changing operand from immediate to constant pool and vice versa.
In the following tests, SUB instruction can be CSEd with CMP, but it's not:

define i256 @test_small_large_imm_ule(i256 %a, i1 %cond) {
  %add = add i256 %a, -65535
  %cmp = icmp ule i256 %a, 65535
  %select1 = select i1 %cmp, i256 %add, i256 %a
  %select2 = select i1 %cond, i256 %select1, i256 %add
  ret i256 %select2
}
define i256 @test_small_large_imm_uge(i256 %a, i1 %cond) {
  %add = add i256 %a, -65536
  %cmp = icmp uge i256 %a, 65536
  %select1 = select i1 %cmp, i256 %add, i256 %a
  %select2 = select i1 %cond, i256 %select1, i256 %add
  ret i256 %select2
}