[AArch64] Use @ pattern to eliminate switch statement in one more place

[AArch64] Use @ pattern to eliminate switch statement in one more place

We can use the new '@' modifier to MD patterns to generate mode-parametrised gen_ functions
for the despeculate_copy patterns and therefore remove the switch on modes in aarch64.c
that is used to pick the right generator. This simplifies the code.

	* config/aarch64/aarch64.md (despeculate_copy<ALLI_TI:mode>):
	Rename to...
	(@despeculate_copy<ALLI_TI:mode>): ... This.
	* config/aarch64/aarch64.c (aarch64_speculation_safe_value): Remove
	switch statement.

From-SVN: r265210