tcg/i386: Conditionalize tcg_out_extu_i32_i64

Since TCG_TYPE_I32 values are kept zero-extended in registers, via
omission of the REXW bit, we need not extend if the register matches.
This is already relied upon by qemu_{ld,st}.

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