cmd/compile: desugar ORECOVER during typecheck

cmd/compile: desugar ORECOVER during typecheck

This never belonged in escape analysis, but the non-unified generics
frontend didn't use typecheck. That frontend is gone, so now we can
desugar it earlier.

Change-Id: I70f34a851f27fce1133777c5eeca0f549fc60ede
Reviewed-on: https://go-review.googlesource.com/c/go/+/518958
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>