The functions Float64bits and Float64frombits perform
poorly on ppc64x because the int<->float conversions
often result in load and store sequences to handle the
type change. This patch adds more rules to recognize
those sequences and use register to register moves
and avoid unnecessary loads and stores where possible.

There were some existing rules to improve these conversions,
but this provides additional improvements. Included here:

- New instruction FCFIDS to improve on conversion to 32 bit
- Rename Xf2i64 and Xi2f64 as MTVSRD, MFVSRD, to match the asm
- Add rules to lower some of the load/store sequences for
- Added new go asm to ppc64.s testcase.
conversions

Improvements:

BenchmarkAbs-16                2.16          0.93          -56.94%
BenchmarkCopysign-16           2.66          1.18          -55.64%
BenchmarkRound-16              4.82          2.69          -44.19%
BenchmarkSignbit-16            1.71          1.14          -33.33%
BenchmarkFrexp-16              11.4          7.94          -30.35%
BenchmarkLogb-16               10.4          7.34          -29.42%
BenchmarkLdexp-16              15.7          11.2          -28.66%
BenchmarkIlogb-16              10.2          7.32          -28.24%
BenchmarkPowInt-16             69.6          55.9          -19.68%
BenchmarkModf-16               10.1          8.19          -18.91%
BenchmarkLog2-16               17.4          14.3          -17.82%
BenchmarkCbrt-16               45.0          37.3          -17.11%
BenchmarkAtanh-16              57.6          48.3          -16.15%
BenchmarkRemainder-16          76.6          65.4          -14.62%
BenchmarkGamma-16              26.0          22.5          -13.46%
BenchmarkPowFrac-16            197           174           -11.68%
BenchmarkMod-16                112           99.8          -10.89%
BenchmarkAsinh-16              59.9          53.7          -10.35%
BenchmarkAcosh-16              44.8          40.3          -10.04%

Updates #21390

Change-Id: I56cc991fc2e55249d69518d4e1ba76cc23904e35
Reviewed-on: https://go-review.googlesource.com/63290Reviewed-by: Michael Munday <mike.munday@ibm.com>