cmd/compile/internal/ssa: lower builtins much later

cmd/compile/internal/ssa: lower builtins much later

* Move lowering into a separate pass.
* SliceLen/SliceCap is now available to various intermediate passes
which use useful for bounds checking.
* Add a second opt pass to handle the new opportunities

Decreases the code size of binaries in pkg/tool/linux_amd64
by ~45K.

Updates #14564 #14606

Change-Id: I5b2bd6202181c50623a3585fbf15c0d6db6d4685
Reviewed-on: https://go-review.googlesource.com/20172
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>