[release-branch.go1.1] cmd/6g, cmd/8g: prevent constant propagation of non-constant LEA.

««« CL 10785043 / cf792c00f410
cmd/6g, cmd/8g: prevent constant propagation of non-constant LEA.

Fixes #5809.

R=golang-dev, dave, rsc, nigeltao
CC=golang-dev
https://golang.org/cl/10785043
»»»

Update #5928

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/11515045