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

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