Previously, genMatch0 and genResult0 contained
lots of duplication: locating the op, parsing
the value, validation, etc.
Parsing and validation was mixed in with code gen.

Extract a helper, parseValue. It is responsible
for parsing the value, locating the op, and doing
shared validation.

As a bonus (and possibly as my original motivation),
make op selection pay attention to the number
of args present.
This allows arch-specific ops to share a name
with generic ops as long as there is no ambiguity.
It also detects and reports unresolved ambiguity,
unlike before, where it would simply always
pick the generic op, with no warning.

Also use parseValue when generating the top-level
op dispatch, to ensure its opinion about ops
matches genMatch0 and genResult0.

The order of statements in the generated code used
to depend on the exact rule. It is now somewhat
independent of the rule. That is the source
of some of the generated code changes in this CL.
See rewritedec64 and rewritegeneric for examples.
It is a one-time change.

The op dispatch switch and functions used to be
sorted by opname without architecture. The sort
now includes the architecture, leading to further
generated code changes.
See rewriteARM and rewriteAMD64 for examples.
Again, it is a one-time change.

There are no functional changes.

Change-Id: I22c989183ad5651741ebdc0566349c5fd6c6b23c
Reviewed-on: https://go-review.googlesource.com/24649
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>