escape: Stack allocate non-escaping expressions.
escape: Stack allocate non-escaping expressions.

Stack allocate expressions that the analysis tracked and determined
did not escape.

Change-Id: Id8f6d5f942ba66feb30f356960d242431496a3b2
Reviewed-on: https://go-review.googlesource.com/22377
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend