cmd/gc: fix order of channel evaluation of receive channels
Normally, an expression of the form x.f or *y can be reordered
with function calls and communications.
Select is stricter than normal: each channel expression is evaluated
in source order. If you have case <-x.f and case <-foo(), then if the
evaluation of x.f causes a panic, foo must not have been called.
(This is in contrast to an expression like x.f + foo().)
Enforce this stricter ordering.
Fixes #8336.
LGTM=dvyukov
R=golang-codereviews, dvyukov
CC=golang-codereviews, r
https://golang.org/cl/
126570043