runtime: shorten hash lookup stack frames
runtime: shorten hash lookup stack frames

On amd64 the frames are very close to the limit for a
nosplit (textflag 7) function, in part because the C compiler
does not make any attempt to reclaim space allocated for
completely registerized variables. Avoid a few short-lived
variables to reclaim two words.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/10758043

tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,tv42/old-go,gnanderson/go,gnanderson/go,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,gnanderson/go,gnanderson/go,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme