target/113255 - avoid REG_POINTER on a pointer difference

The following avoids re-using a register holding a pointer (and
thus might be REG_POINTER) for the result of a pointer difference
computation.  That might confuse heuristics in (broken) RTL alias
analysis which relies on REG_POINTER indicating that we're
dealing with one.

This alone doesn't fix anything.

	PR target/113255
	* config/i386/i386-expand.cc
	(expand_set_or_cpymem_prologue_epilogue_by_misaligned_moves):
	Use a new pseudo for the skipped number of bytes.