match: Remove `:c` in mul_cross_sum

mul_cross_sum is just matching a simple plus expression without any conditions
so it does not need the `:c` on the plus.

This reduces the produced code from genmatch slightly but has no effect on the
outcome as there is just a plain return there before the check of the opposite
operands.

Pushed as obvious after a bootstrap/test on x86_64-linux-gnu.

gcc/ChangeLog:

	* match.pd (mul_cross_sum): Remove the `:c` from the plus.

Signed-off-by: Andrea Pinski <andrew.pinski@oss.qualcomm.com>