cmd/gc, runtime: optimize map[string] lookup from []byte key
Brad has been asking for this for a while.
I have resisted because I wanted to find a more general way to
do this, one that would keep the performance of code introducing
variables the same as the performance of code that did not.
(See golang.org/issue/3512#c20).
I have not found the more general way, and recent changes to
remove ambiguously live temporaries have blown away the
property I was trying to preserve, so that's no longer a reason
not to make the change.
Fixes #3512.
LGTM=iant
R=iant
CC=bradfitz, golang-codereviews, khr, r
https://golang.org/cl/
83740044