[SDAG] fold sub-of-shift to add-of-shift

This fold is done in IR:
https://alive2.llvm.org/ce/z/jWyFrP

There is an x86 test that shows an improvement
from the added flexibility of using add (commutative).

The other diffs are presumed neutral.

Note that this could also be folded to an 'xor',
but I'm not sure if that would be universally better
(eg, x86 can convert adds more easily into LEA).

This helps prevent regressions from a potential fold for
issue #53829.