[LoopUnroll] Adjust CostKind query

When TTI was updated to use an explicit cost, TCK_CodeSize was used
although the default implicit cost would have been the hand-wavey
cost of size and latency. So, revert back to this behaviour. This is
not expected to have (much) impact on targets since most (all?) of
them return the same value for SizeAndLatency and CodeSize.

When optimising for size, the logic has been changed to query
CodeSize costs instead of SizeAndLatency.

This patch also adds a testing option in the unroller so that
OptSize thresholds can be specified.

Differential Revision: https://reviews.llvm.org/D85723