[dev.ssa] cmd/compile/internal/ssa: implement lots of small (<8byte) ops.

[dev.ssa] cmd/compile/internal/ssa: implement lots of small (<8byte) ops.

Lots and lots of ops!
Also XOR for good measure.

Add a pass to the compiler generator to check that all of the
architecture-specific opcodes are handled by genValue.  We will
catch any missing ones if we come across them during compilation,
but probably better to catch them statically.

Change-Id: Ic4adfbec55c8257f88117bc732fa664486262868
Reviewed-on: https://go-review.googlesource.com/12813
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>