target/i386: reimplement 0x0f 0x50-0x5f, add AVX

target/i386: reimplement 0x0f 0x50-0x5f, add AVX

These are mostly floating-point SSE operations.  The odd ones out
are MOVMSK and CVTxx2yy, the others are straightforward.

Unary operations are a bit special in AVX because they have 2 operands
for PD/PS operands (VEX.vvvv must be 1111b), and 3 operands for SD/SS.
They are handled using X86_OP_GROUP3 for compactness.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>