tcg/optimize: fold constant test in tcg_opt_gen_mov

Most of the calls to tcg_opt_gen_mov are preceeded by a test to check if
the source temp is a constant. Fold that into the tcg_opt_gen_mov
function.

Cc: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Message-Id: <1433495958-9508-1-git-send-email-aurelien@aurel32.net>
Signed-off-by: Richard Henderson <rth@twiddle.net>
