These *almost* match what cc65 has, and are accepted as primary or
aliases by 64tass.
This combines the LAX and LXA operations. LXA is the immediate
form of LAX, and behaves somewhat differently (and is unstable).
I was treating them as two separate operations with independent
mnemonics, but that doesn't seem to be the preferred way to
handle it.
The cc65 generator wasn't generating LAX before; now it does. This
required nudging the width disambiguator, as LAX is a second
example of an instruction with both DP,Y and ABS,Y operands.
(issue #20)
