cmd/compile: improve lowered moves and zeros for ppc64le
This change includes the following:
- Generate LXV/STXV sequences instead of LXVD2X/STXVD2X on power9.
These instructions do not require an index register, which
allows more loads and stores within a loop without initializing
multiple index registers. The LoweredQuadXXX generate LXV/STXV.
- Create LoweredMoveXXXShort and LoweredZeroXXXShort for short
moves that don't generate loops, and therefore don't clobber the
address registers or flags.
- Use registers other than R3 and R4 to avoid conflicting with
registers that have already been allocated to avoid unnecessary
register moves.
- Eliminate the use of R14 as scratch register and use R31
instead.
- Add PCALIGN when the LoweredMoveXXX or LoweredZeroXXX generates a
loop with more than 3 iterations.
This performance opportunity was noticed in github.com/golang/snappy
benchmarks. Results on power9: