I created a .s file that covered every instruction and operand production
in 9a/a.y and made sure that 9a and asm give bit-identical results for it.
I found a few things, including one addressing mode (R1+R2) that was
not present in the source we use. Fixed those

I also found quite a few things where 9a's grammar accepts the instruction
but liblink rejects it. These need to be sorted out, and I will do that separately.
Once that's done, I'll turn my test file into a proper test.

Change-Id: Ib093271b0f7ffd64ffed164ed2a820ebf2420e34
Reviewed-on: https://go-review.googlesource.com/5361Reviewed-by: Russ Cox <rsc@golang.org>