target-i386: fix translation of sse {, u}comis{s, d} instructions

While the generic SSE translation codepath contains special logic to use
32-bit or 64-bit memory operands for some instructions, this logic doesn't
catch the SSE {,u}comis{s,d} instructions.  This oversight leads to too
many bytes being read when those instructions use memory operands, which
can in turn lead to page faults.

The fix is simple: add a special case for these instructions.  It did not
fit cleanly into the existing case, so some cut-and-paste was necesary.

Signed-off-by: Nathan Froyd <froydnj@mozilla.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>