tcg: Adjust simd_desc size encoding

tcg: Adjust simd_desc size encoding

With larger vector sizes, it turns out oprsz == maxsz, and we only
need to represent mismatch for oprsz <= 32.  We do, however, need
to represent larger oprsz and do so without reducing SIMD_DATA_BITS.

Reduce the size of the oprsz field and increase the maxsz field.
Steal the oprsz value of 24 to indicate equality with maxsz.

Tested-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>