[x86] refine conditions for immediate hoisting to save code-size

As shown in PR46237:
https://bugs.llvm.org/show_bug.cgi?id=46237

The size-savings win for hoisting an 8-bit ALU immediate (intentionally
excluding store constants) requires extreme conditions; it may not even
be possible when including REX prefix bytes on x86-64.

I did draft a version of this patch that included use counts after the
loop, but I suspect that accounting is not working as expected. I think
that is because the number of constant uses are changing as we select
instructions (for example as we transform shl/add into LEA).

Differential Revision: https://reviews.llvm.org/D81468