Apart from reducing the number of LEAL/LEAQ instructions by about
30%, it gives 8g easier registerization in several cases,
for example in strconv. Performance with 6g is not affected.

Before (386):
src/pkg/strconv/decimal.go:22   TEXT  (*decimal).String+0(SB),$240-12
src/pkg/strconv/extfloat.go:540 TEXT  (*extFloat).ShortestDecimal+0(SB),$584-20

After (386):
src/pkg/strconv/decimal.go:22   TEXT  (*decimal).String+0(SB),$196-12
src/pkg/strconv/extfloat.go:540 TEXT  (*extFloat).ShortestDecimal+0(SB),$420-20

Benchmarks with GOARCH=386 (on a Core 2).

benchmark                 old ns/op    new ns/op    delta
BenchmarkBinaryTree17    7110191000   7079644000   -0.43%
BenchmarkFannkuch11      7769274000   7766514000   -0.04%
BenchmarkGobDecode         33454820     34755400   +3.89%
BenchmarkGobEncode         11675710     11007050   -5.73%
BenchmarkGzip            2013519000   1593855000  -20.84%
BenchmarkGunzip           253368200    242667600   -4.22%
BenchmarkJSONEncode       152443900    120763400  -20.78%
BenchmarkJSONDecode       304112800    247461800  -18.63%
BenchmarkMandelbrot200     29245520     29240490   -0.02%
BenchmarkParse              8484105      8088660   -4.66%
BenchmarkRevcomp         2695688000   2841263000   +5.40%
BenchmarkTemplate         363759800    277271200  -23.78%

benchmark                       old ns/op    new ns/op    delta
BenchmarkAtof64Decimal                127          129   +1.57%
BenchmarkAtof64Float                  166          164   -1.20%
BenchmarkAtof64FloatExp               308          300   -2.60%
BenchmarkAtof64Big                    584          571   -2.23%
BenchmarkAppendFloatDecimal           440          430   -2.27%
BenchmarkAppendFloat                  995          776  -22.01%
BenchmarkAppendFloatExp               897          746  -16.83%
BenchmarkAppendFloatNegExp            900          752  -16.44%
BenchmarkAppendFloatBig              1528         1228  -19.63%
BenchmarkAppendFloat32Integer         443          453   +2.26%
BenchmarkAppendFloat32ExactFraction   812          661  -18.60%
BenchmarkAppendFloat32Point          1002          773  -22.85%
BenchmarkAppendFloat32Exp             858          725  -15.50%
BenchmarkAppendFloat32NegExp          848          728  -14.15%
BenchmarkAppendFloat64Fixed1          447          431   -3.58%
BenchmarkAppendFloat64Fixed2          480          462   -3.75%
BenchmarkAppendFloat64Fixed3          461          457   -0.87%
BenchmarkAppendFloat64Fixed4          509          484   -4.91%

Update #1914.

R=rsc, nigeltao
CC=golang-dev, remy
https://golang.org/cl/6494107