Inlining of switch statements into a RETURNed expression
can sometimes lead to the switch being walked twice, which
results in a miscompiled switch statement. The bug depends
on:

1) multiple results
2) named results
3) a return statement whose expression includes a call to a
function containing a switch statement that is inlined.

It may also be significant that the default case of that
switch is a panic(), though that's not proven.

Rearranged the walk case for ORETURN so that double walks are
not possible.  Added a test, because this is so fiddly.
Added a check against double walks, verified that it fires
w/o other fix.

Fixes #25776.

Change-Id: I2d594351fa082632512ef989af67eb887059729b
Reviewed-on: https://go-review.googlesource.com/118318
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>