cmd/compile: allow naming of subexpressions

cmd/compile: allow naming of subexpressions

Allow names to be used for subexpressions of match rules.
For example:

(OpA x:(OpB y)) -> ..use x here to refer to the OpB value..

This gets rid of the .Args[0].Args[0]... way of naming we
used to use.

While we're here, give all subexpression matches names instead
of recomputing them with .Args[i] sequences each time they
are referenced.  Makes the generated rule code a bit smaller.

Change-Id: Ie42139f6f208933b75bd2ae8bd34e95419bc0e4e
Reviewed-on: https://go-review.googlesource.com/20997
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Todd Neal <todd@tneal.org>