[AArch64] PR target/84748: Mark *compare_cstore<mode>_insn as clobbering CC reg

[AArch64] PR target/84748: Mark *compare_cstore<mode>_insn as clobbering CC reg

In this wrong-code PR the combine pass ends up moving a CC-using instruction past a *compare_cstore<mode>_insn
insn_and_split. After reload the *compare_cstore<mode>_insn splitter ends up generating a SUBS instruction that
clobbers the condition flags, and things go bad.

The solution is simple, the *compare_cstore<mode>_insn pattern should specify that it clobbers the CC register
so that combine (or any other pass) does not assume that it can move CC-using patterns across it.

This patch does that and fixes the testcase.

The testcase FAILs on GCC 8 only, but the buggy pattern is in GCC 6 onwards, so we should backport this as
a latent bug fix after it's had some time to bake in trunk.

Bootstrapped and tested on aarch64-none-linux-gnu.

	PR target/84748
	* config/aarch64/aarch64.md (*compare_cstore<mode>_insn): Mark pattern
	as clobbering CC_REGNUM.

	* gcc.c-torture/execute/pr84748.c: New test.

From-SVN: r258366