aarch64: Improve vcombine codegen [PR89057]

aarch64: Improve vcombine codegen [PR89057]

This patch fixes a codegen regression in the handling of things like:

  __temp.val[0]								     \
    = vcombine_##funcsuffix (__b.val[0],				     \
			     vcreate_##funcsuffix (__AARCH64_UINT64_C (0))); \

in the 64-bit vst[234] functions.  The zero was forced into a
register at expand time, and we relied on combine to fuse the
zero and combine back together into a single combinez pattern.
The problem is that the zero could be hoisted before combine
gets a chance to do its thing.

gcc/
	PR target/89057
	* config/aarch64/aarch64-simd.md (aarch64_combine<mode>): Accept
	aarch64_simd_reg_or_zero for operand 2.  Use the combinez patterns
	to handle zero operands.

gcc/testsuite/
	PR target/89057
	* gcc.target/aarch64/pr89057.c: New test.