aarch64: Fix ICE when expanding scalar floating move with -mgeneral-regs-only. [PR94991]

aarch64: Fix ICE when expanding scalar floating move with -mgeneral-regs-only. [PR94991]

In the testcase for PR94991, we are doing FAIL for scalar floating move expand
pattern since TARGET_FLOAT is false with option -mgeneral-regs-only. But move
expand pattern cannot fail. It would be better to replace the FAIL with code
that bitcasts to the equivalent integer mode using gen_lowpart.

2020-05-11  Felix Yang  <felix.yang@huawei.com>

gcc/
	PR target/94991
	* config/aarch64/aarch64.md (mov<mode>):
	Bitcasts to the equivalent integer mode using gen_lowpart
	instead of doing FAIL for scalar floating point move.

gcc/testsuite/
	PR target/94991
	* gcc.target/aarch64/mgeneral-regs_5.c: New test.