rtl: directly handle MEM in gen_highpart [PR102125]

rtl: directly handle MEM in gen_highpart [PR102125]

gen_lowpart_general handles forming a lowpart of a MEM by using
adjust_address to rework and validate a new version of the MEM.
Do the same for gen_highpart rather than calling simplify_gen_subreg
for this case.

gcc/ChangeLog:

	PR target/102125
	* emit-rtl.c (gen_highpart): Use adjust_address to handle
	MEM rather than calling simplify_gen_subreg.