MIPS64: small improvements in code generation

Specifically:
- More efficient load/store of constant 0 (and +0.0)
- Improved swapping of floats/doubles in registers
- Use kNoOutputOverlap wherever possible
- More efficient 64-bit integer comparison with 0
- More efficient load of integer constants of the form (2**n)-1

Change-Id: Ic2914d8865aa6616b9a0b21b3cc173d4477eb8c7
2 files changed