[lra] take scratch as implicit unused output reloads [PR55212]

[lra] take scratch as implicit unused output reloads [PR55212]

When trying to convert the SH port to use LRA, the first issue I hit
was the need for dealing with former scratch registers at places we
didn't need to on other ports, treating them like unused output
reloads instead of rejecting them.

for  gcc/ChangeLog

	PR target/55212
	* lra-constraints.cc (match_reload): Treat former scratch
	regs as implicit unused output reloads.
	(process_alt_operands): Likewise.
	(curr_insn_transform): Likewise.