cmd/compile: eliminate Prog-related globals

cmd/compile: eliminate Prog-related globals

Introduce a new type, gc.Progs, to manage
generation of Progs for a function.
Use it to replace globals pc and pcloc.

Passes toolstash-check -all.

Updates #15756

Change-Id: I2206998d7c58fe2a76b620904909f2e1cec8a57d
Reviewed-on: https://go-review.googlesource.com/38418
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>