tcg/i386: Implement bitsel for avx512

The general ternary logic operation can implement BITSEL.
Funnel the 4-operand operation into three variants of the
3-operand instruction, depending on input operand overlap.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>