cmd/gc: save local var list before inlining

cmd/gc: save local var list before inlining

This avoids problems with inlining in genwrappers, which
occurs after functions have been compiled.  Compiling a
function may cause some unused local vars to be removed from
the list.  Since a local var may be unused due to
optimization, it is possible that a removed local var winds up
beingused in the inlined version, in which case hilarity
ensues.

Fixes #5515.

R=golang-dev, khr, dave
CC=golang-dev
https://golang.org/cl/10210043