This is the gofrontend version of https://golang.org/cl/245123.
Original CL description:
Currently, selectgo does an initial pass over the cases array to look
for entries with nil channels, so they can be easily recognized and
skipped later on. But this still involves actually visiting the cases.
This commit changes selectgo to omit cases with nil channels when
constructing pollorder, so that they'll be skipped over entirely later
on. It also checks for caseDefault up front, which will facilitate
changing it to use a "block bool" parameter instead.
Updates golang/go#40410
This is being brought over to gofrontend as a step toward upgrading to
Go1.16beta1, setting up for more compiler changes related to select handling.
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/279733
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