tcg: Split out tcg_out_extrl_i64_i32

tcg: Split out tcg_out_extrl_i64_i32

We will need a backend interface for type truncation.  For those backends
that did not enable TCG_TARGET_HAS_extrl_i64_i32, use tcg_out_mov.
Use it in tcg_reg_alloc_op in the meantime.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>