cmd/gc: escape analysis to track flow of in to out parameters.

cmd/gc: escape analysis to track flow of in to out parameters.

includes step 0: synthesize outparams, from 6600044
step 1: give outparams loopdepth 0 and verify unchanged results
step 2: generate esc:$mask tags, but still tie to sink if a param has mask != 0
next step: use in esccall (and ORETURN with implicit OAS2FUNC) to avoid tying to sink

R=rsc
CC=golang-dev
https://golang.org/cl/6610054