[LV] Count cost of middle block if TC <= VF. (#168949)

If the expected trip count is less than the VF, the vector loop will
only execute a single iteration. When that's the case, the cost of the
middle block has the same impact as the cost of the vector loop. Include
it in isOutsideLoopWorkProfitable to avoid vectorizing when the extra
work in the middle block makes it unprofitable.

Note that isOutsideLoopWorkProfitable already scales the cost of blocks
outside the vector region, but the patch restricts accounting for the
middle block to cases where VF <= ExpectedTC, to initially catch some
worst cases and avoid regressions.

This initial version should specifically avoid unprofitable tail-folding
for loops with low trip counts after re-applying
https://github.com/llvm/llvm-project/pull/149042.

PR: https://github.com/llvm/llvm-project/pull/168949