tree-optimization/109672 - properly check emulated plus during vect

tree-optimization/109672 - properly check emulated plus during vect

The following refactors the check for emulated vector support for
the cases of plus, minus and negate.  In the PR we end up with
a SImode plus, supported by the target but emulated and in this
context fail to verify we are dealing with exactly word_mode.

	PR tree-optimization/109672
	* tree-vect-stmts.cc (vectorizable_operation): For plus,
	minus and negate always check the vector mode is word mode.