cmd/8g: extend elimination of temporaries to SSE2 code.

Before:
(erf.go:188)    TEXT     Erf+0(SB),$220
(erf.go:265)    TEXT     Erfc+0(SB),$204
(lgamma.go:174) TEXT     Lgamma+0(SB),$948

After:
(erf.go:188)    TEXT     Erf+0(SB),$84
(erf.go:265)    TEXT     Erfc+0(SB),$84
(lgamma.go:174) TEXT     Lgamma+0(SB),$44

SSE before vs. SSE after:

benchmark             old ns/op    new ns/op    delta
BenchmarkAcosh               81           49  -39.14%
BenchmarkAsinh              109          109   +0.00%
BenchmarkAtanh               73           74   +0.68%
BenchmarkLgamma             138           42  -69.20%
BenchmarkModf                24           15  -36.95%
BenchmarkSqrtGo             565          556   -1.59%

R=rsc
CC=golang-dev
https://codereview.appspot.com/7028048