target/i386: Use assert() to sanity-check b1 in SSE decode

target/i386: Use assert() to sanity-check b1 in SSE decode

In the SSE decode function gen_sse(), we combine a byte
'b' and a value 'b1' which can be [0..3], and switch on them:
   b |= (b1 << 8);
   switch (b) {
   ...
   default:
   unknown_op:
       gen_unknown_opcode(env, s);
       return;
   }

In three cases inside this switch, we were then also checking for
 "if (b1 >= 2) { goto unknown_op; }".
However, this can never happen, because the 'case' values in each place
are 0x0nn or 0x1nn and the switch will have directed the b1 == (2, 3)
cases to the default already.

This check was added in commit c045af25a52e9 in 2010; the added code
was unnecessary then as well, and was apparently intended only to
ensure that we never accidentally ended up indexing off the end
of an sse_op_table with only 2 entries as a result of future bugs
in the decode logic.

Change the checks to assert() instead, and make sure they're always
immediately before the array access they are protecting.

Fixes: Coverity CID 1460207
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>