[i386] Reject too large vectors for partial vector vectorization

[i386] Reject too large vectors for partial vector vectorization

The following works around the lack of the x86 backend making the
vectorizer compare the costs of the different possible vector
sizes the backed advertises through the vector_modes hook.  When
enabling masked epilogues or main loops then this means we will
select the prefered vector mode which is usually the largest even
for loops that do not iterate close to the times the vector has
lanes.  When not using masking the vectorizer would reject any
mode resulting in a VF bigger than the number of iterations
but with masking they are simply masked out.

So this overloads the finish_cost function and matches for
the problematic case, forcing a high cost to make us try a
smaller vector size.

	* config/i386/i386.cc (ix86_vector_costs::finish_cost):
	Overload.  For masked main loops make sure the vectorization
	factor isn't more than double the number of iterations.

	* gcc.target/i386/vect-partial-vectors-1.c: New testcase.
	* gcc.target/i386/vect-partial-vectors-2.c: Likewise.