add instr_replace_reg() which covers overlapping regs and resizes to match

Today we have opnd_replace_reg(), which only replaces an exact match, and does not resize to match (as it is typically used to replace one reg with a differently sized version of itself).

This is a request for an instr-level routine to replace which cover overlapping regs and resizes to match. This would initially be used for #1698. I would want to be able to say "replace x28 with x0" and have it modify strex w28, x1, [x2] to become strex w0, x1, [x2].