[InstCombine] improve demanded bits for Sub operand 0

This is copying the code that was added for 'add' with D130075.
(That patch removed a fallthrough in the cases, but we can
probably still share at least some code again as a follow-up
cleanup, but I didn't want to risk it here.)

The reasoning is similar to the carry propagation for 'add':
if we don't demand low bits of the subtraction and the
subtrahend (aka RHS or operand 1) is known zero in those low
bits, then there can't be any borrowing required from the
higher bits of operand 0, so the low bits don't matter.

Also, the no-wrap flags can be propagated (and I think that
should be true for add too).

Here's an attempt to prove that in Alive2:
https://alive2.llvm.org/ce/z/xqh7Pa
(can add nsw or nuw to src and tgt, and it should still pass)

Differential Revision: https://reviews.llvm.org/D136788