cmd/compile: improve LoweredZero performance for ppc64x

cmd/compile: improve LoweredZero performance for ppc64x

This change improves the performance of the LoweredZero rule
on ppc64x.

The improvement can be seen in the runtime ClearFat
benchmarks:

BenchmarkClearFat12-16       2.40          0.69          -71.25%
BenchmarkClearFat16-16       9.98          0.93          -90.68%
BenchmarkClearFat24-16       4.75          0.93          -80.42%
BenchmarkClearFat32-16       6.02          0.93          -84.55%
BenchmarkClearFat40-16       7.19          1.16          -83.87%
BenchmarkClearFat48-16       15.0          1.39          -90.73%
BenchmarkClearFat56-16       9.95          1.62          -83.72%
BenchmarkClearFat64-16       18.0          1.86          -89.67%
BenchmarkClearFat128-16      30.0          8.08          -73.07%
BenchmarkClearFat256-16      52.5          11.3          -78.48%
BenchmarkClearFat512-16      97.0          19.0          -80.41%
BenchmarkClearFat1024-16     244           34.2          -85.98%

Fixes: #19532
Change-Id: If493e28bc1d8e61bc79978498be9f5336a36cd3f
Reviewed-on: https://go-review.googlesource.com/38096
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <munday@ca.ibm.com>