Currently when considering register pressure is enabled, we reject any
VF that has higher pressure than the number of registers. However this
can result in failing to vectorize in cases where it's beneficial, as
the cost of the extra spills is less than the benefit we get from
vectorizing.
Deal with this by instead calculating the cost of spills and adding that
to the rest of the cost, so we can detect this kind of situation and
still vectorize while avoiding vectorizing in cases where the extra cost
makes it not with it.
a083e19efe