tcg: Convert extrl_i64_i32 to TCGOutOpUnary

tcg: Convert extrl_i64_i32 to TCGOutOpUnary

Drop the cast from TCGv_i64 to TCGv_i32 in tcg_gen_extrl_i64_i32
an emit extrl_i64_i32 unconditionally.  Move that special case
to tcg_gen_code when we find out if the output is live or dead.
In this way even hosts that canonicalize truncations can make
use of a store directly from the 64-bit host register.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>