tcg/riscv: Conditionalize tcg_out_exts_i32_i64

Since TCG_TYPE_I32 values are kept sign-extended in registers, via "w"
instructions, we don't need to extend if the register matches.
This is already relied upon by comparisons.

Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>