cmd/compile: convert type during finishcompare

cmd/compile: convert type during finishcompare

When recursively calling walkexpr, r.Type is still the untyped value.
It then sometimes recursively calls finishcompare, which complains that
you can't compare the resulting expression to that untyped value.

Updates #23834.

Change-Id: I6b7acd3970ceaff8da9216bfa0ae24aca5dee828
Reviewed-on: https://go-review.googlesource.com/97856
Reviewed-by: Matthew Dempsky <mdempsky@google.com>