fwprop: Don't propagate asms [PR121253]

For the reasons explained in the comment, fwprop shouldn't even
try to propagate an asm definition.

gcc/
	PR rtl-optimization/121253
	* fwprop.cc (forward_propagate_into): Don't propagate asm defs.

gcc/testsuite/
	PR rtl-optimization/121253
	* gcc.target/aarch64/pr121253.c: New test.

(cherry picked from commit e82c8413eda498163ae2e0ecc458ea0428708c30)
2 files changed