cmd/compile: add more ARM64 optimizations

- Use machine instructions for uint64<->float conversions
- Do not enforce alignment on Zero/Move
	ARM64 supports unaligned load/stores, but only aligned offset
	or small offset can be encoded into instructions.
- Do combined loads

Change-Id: Iffca7dd0f13070b17b784861ce5a30af584680eb
Reviewed-on: https://go-review.googlesource.com/27086


Reviewed-by: David Chase <drchase@google.com>