aarch64: force operand to fresh register to avoid subreg issues [PR118892]

aarch64: force operand to fresh register to avoid subreg issues [PR118892]

When the input is already a subreg and we try to make a paradoxical
subreg out of it for copysign this can fail if it violates the subreg
relationship.

Use force_lowpart_subreg instead of lowpart_subreg to then force the
results to a register instead of ICEing.

gcc/ChangeLog:

	PR target/118892
	* config/aarch64/aarch64.md (copysign<GPF:mode>3): Use
	force_lowpart_subreg instead of lowpart_subreg.

gcc/testsuite/ChangeLog:

	PR target/118892
	* gcc.target/aarch64/copysign-pr118892.c: New test.