tcg: Add support for vector compare select

tcg: Add support for vector compare select

Perform a per-element conditional move.  This combination operation is
easier to implement on some host vector units than plain cmp+bitsel.
Omit the usual gvec interface, as this is intended to be used by
target-specific gvec expansion call-backs.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>