The "test" instruction is used in very limited situations. I've made a best effort
to fill in the possible forms (address for the first operand), but it's not tested,
so I put the *untested* parts behind an assert. Otherwise it's very similar to
icmp, so if it starts to be used and tested then the asserts can be taken out,
and the code shared with icmp.

Tighten some of the XMM dispatch/emitters. Most of those XMM instructions
can only encode the variant where dest is a register. Rather than waste
a slot for a NULL method pointer, just make the struct type have two variants
instead of three.

Fill out a couple of XMM instructions which *do* allow mem-ops as dest
(mov instructions).

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/624263002