cmd/compile: remove gins

cmd/compile: remove gins

The only remaining consumers of gins were
ginsnop and arch-independent opcodes like GVARDEF.
Rewrite ginsnop to create and populate progs directly.
Move arch-independent opcodes to package gc
and simplify.
Delete some now unused code.
There is more.
Step one towards eliminating gc.Node.Reg.

Change-Id: I7c34cd8a848f6fc3b030705ab8e293838e0b6c20
Reviewed-on: https://go-review.googlesource.com/29220
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>