target/m68k: Avoid tcg_const_i32 in bfop_reg

target/m68k: Avoid tcg_const_i32 in bfop_reg

Tidy up the whole function, hoisting is_bfffo as a common test
for whether tlen and tofs needed. Use tcg_constant_i32, and load
a separate temporary for mask.

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