aarch64: Simplify LSE cas generation

The cas insn is a single insn, and if expanded properly need not
be split after reload.  Use the proper inputs for the insn.

	* config/aarch64/aarch64.c (aarch64_expand_compare_and_swap):
	Force oldval into the rval register for TARGET_LSE; emit the compare
	during initial expansion so that it may be deleted if unused.
	(aarch64_gen_atomic_cas): Remove.
	* config/aarch64/atomics.md (@aarch64_compare_and_swap<SHORT>_lse):
	Change =&r to +r for operand 0; use match_dup for operand 2;
	remove is_weak and mod_f operands as unused.  Drop the split
	and merge with...
	(@aarch64_atomic_cas<SHORT>): ... this pattern's output; remove.
	(@aarch64_compare_and_swap<GPI>_lse): Similarly.
	(@aarch64_atomic_cas<GPI>): Similarly.

From-SVN: r265656