cmd/5g: avoid temporaries in agen OINDEX
Most benchmarks are within the 3% margin of error. This code path is quite common in the fmt package.
linux/arm, Freescale iMX.53 (cortex-a8)
fmt:
benchmark old ns/op new ns/op delta
BenchmarkSprintfEmpty 925 785 -15.14%
BenchmarkSprintfString 5050 5039 -0.22%
BenchmarkSprintfInt 4425 4406 -0.43%
BenchmarkSprintfIntInt 5802 5762 -0.69%
BenchmarkSprintfPrefixedInt 7029 6541 -6.94%
BenchmarkSprintfFloat 10278 10083 -1.90%
BenchmarkManyArgs 18558 17606 -5.13%
BenchmarkScanInts
15592690 15415360 -1.14%
BenchmarkScanRecursiveInt
25155020 25050900 -0.41%
R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/
6921056