compiler: Fix order of evalation for m[0] = len(m) when m is a map.
compiler: Fix order of evalation for m[0] = len(m) when m is a map.

R=iant
CC=gofrontend-dev
https://golang.org/cl/6113045

anlhord/gofrontend,golang/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend,golang/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,golang/gofrontend,golang/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,anlhord/gofrontend