AArch64: Improve ctz and ffs

AArch64: Improve ctz and ffs

Use the ctz insn in the ffs expansion so it uses ctz if CSSC
is available. Rather than splitting, keep ctz as a single
insn for simplicity and possible fusion opportunities.
Move clz, ctz, clrsb, rbit and ffs instructions together.

gcc:
	* config/aarch64/aarch64.md (ffs<mode>2): Use gen_ctz.
	(ctz<mode>2): Model ctz as a single target instruction.

gcc/testsuite:
	* gcc.target/aarch64/ffs.c: Improve test.