cmd/gc: fix two select temporary bugs

The introduction of temporaries in order.c was not
quite right for two corner cases:

1) The rewrite that pushed new variables on the lhs of
a receive into the body of the case was dropping the
declaration of the variables. If the variables escape,
the declaration is what allocates them.
Caught by escape analysis sanity check.
In fact the declarations should move into the body
always, so that we only allocate if the corresponding
case is selected. Do that. (This is an optimization that
was already present in Go 1.2. The new order code just
made it stop working.)

Fixes issue 7997.

2) The optimization to turn a single-recv select into
an ordinary receive assumed it could take the address
of the destination; not so if the destination is _.

Fixes issue 7998.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/100480043

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