aarch64: Prevent generation of /M BRKAS and BRKBS

Bit of a brown-paper-bag bug, but: GCC was generating
non-existent merging forms of BRKAS and BRKBS.  Those
instructions only support zero predication (although
BRKA and BRKB support both).

gcc/
	* config/aarch64/aarch64-sve.md (*aarch64_brk<brk_op>_cc): Remove
	merging alternative.
	(*aarch64_brk<brk_op>_ptest): Likewise.

gcc/testsuite/
	* gcc.target/aarch64/sve/acle/general/brka_1.c: Expect a separate
	PTEST instruction.
	* gcc.target/aarch64/sve/acle/general/brkb_1.c: Likewise.