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 issue 8336.

LGTM=dvyukov
R=golang-codereviews, dvyukov
CC=golang-codereviews, r
https://codereview.appspot.com/126570043