cmd/internal/gc: cache ProgInfo in Prog

cmd/internal/gc: cache ProgInfo in Prog

The ProgInfo is loaded many times during each analysis pass.
Load it once at the beginning (in Flowstart if using that, or explicitly,
as in plive.go) and then refer to the cached copy.

Removes many calls to proginfo.

Makes Prog a little bigger, but the previous CL more than compensates.

Change-Id: If90a12fc6729878fdae10444f9c3bedc8d85026e
Reviewed-on: https://go-review.googlesource.com/7745
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>