PR target/108589 - Check REG_P for AARCH64_FUSE_ADDSUB_2REG_CONST1

PR target/108589 - Check REG_P for AARCH64_FUSE_ADDSUB_2REG_CONST1

This adds a check for REG_P on SET_DEST for the new idiom recognizer
for AARCH64_FUSE_ADDSUB_2REG_CONST1.  The reported ICE is only
observable with checking=rtl.

Bootstrapped/regtested aarch64-linux, committed.

	PR target/108589

gcc/ChangeLog:

	* config/aarch64/aarch64.cc (aarch_macro_fusion_pair_p): Check
	REG_P on SET_DEST.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/pr108589.c: New test.