cmd/internal/gc: more Node cleanups

cmd/internal/gc: more Node cleanups

More cleanups to gc.Node

- make Node.Local a boolean
- make Type.Local a boolean
- reduce the size of Node.Esc to a uint8

Reducing the size of Node.Esc shaves ~45mb off the RSS compiling cmd/internal/gc on amd64

before:
        Maximum resident set size (kbytes): 659496
after:
        Maximum resident set size (kbytes): 612196

- declare gc.Funcdepth as int32
- declare Node.Funcdepth as int32

In both cases, these were previously machine specific int types. This doesn't result in
any memory saving at the moment due to struct padding.

Change-Id: Iabef8da15e962fe8b79d7fd3d402fb26ce7ec31c
Reviewed-on: https://go-review.googlesource.com/7261
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Dave Cheney <dave@cheney.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>