[release-branch.go1.1] cmd/gc: avoid passing unevaluated constant expressions to backends.

««« CL 11107044 / 5baf6060648e
cmd/gc: avoid passing unevaluated constant expressions to backends.

Backends do not exactly expect receiving binary operators with
constant operands or use workarounds to move them to
register/stack in order to handle them.

Fixes #5841.

R=golang-dev, daniel.morsing, rsc
CC=golang-dev
https://golang.org/cl/11107044

»»»

Update #5928

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/11879044