aarch64: Fix invalid subregs in xorsign [PR118501]

aarch64: Fix invalid subregs in xorsign [PR118501]

In the testcase, we try to use xorsign on:

   (subreg:DF (reg:TI R) 8)

i.e. the highpart of the TI.  xorsign wants to take a V2DF
paradoxical subreg of this, which is rightly rejected as a direct
operation.  In cases like this, we need to force the highpart into
a fresh register first.

gcc/
	PR target/118501
	* config/aarch64/aarch64.md (@xorsign<mode>3): Use
	force_lowpart_subreg.

gcc/testsuite/
	PR target/118501
	* gcc.c-torture/compile/pr118501.c: New test.