[LoopVectorize] Use CodeSize as the cost kind for minsize (#124119)
Functions marked with minsize should aim for minimum code size, so the
vectorizer should use CodeSize for the cost kind and also the cost we
compare should be the cost for the entire loop: it shouldn't be divided
by the number of vector elements and block costs shouldn't be divided by
the block probability.
Possibly we should also be doing this for optsize as well, but there are
a lot of tests that assume the current behaviour and the definition of
optsize is less clear than minsize (for minsize the goal is to "keep the
code size of this function as small as possible" whereas for optsize
it's "keep the code size of this function low").
