Don't accumulate a massive list of Prog structs during
compilation and write them all out at the end of compilation.
Instead, convert them to code+relocs (or data+relocs) after each
function is compiled.

Track down a few other places that were keeping Progs alive
and nil them out so the Progs get GCd promptly.

Saves ~20% in peak memory usage for the compiler.  Surprisingly not much
help speed-wise (only because we end up doing more GCs.  With a
compensating GOGC=120, it does help a bit), but this provides a base for
more changes (e.g. reusing a cache of Progs).

Change-Id: I838e01017c228995a687a8110d0cd67bf8596407
Reviewed-on: https://go-review.googlesource.com/19867
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>