compiler: fix variable context nit in write barrier generation
compiler: fix variable context nit in write barrier generation

Update the write barrier generation code to insure that the "lvalue
context" tag on the space var expression is set only in the case where
the expr feeds directly into an assignment. This is somewhat
counter-intuitive, but needed in the case where the backend looks at
context tags.

Change-Id: I068d893d68365c97580e3c642f52cb64e15c4b77
Reviewed-on: https://go-review.googlesource.com/43031
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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