[VPlan] Compute interleave count for VPlan. (#149702)

Move selectInterleaveCount to LoopVectorizationPlanner and retrieve some
information directly from VPlan. Register pressure was already computed
for a VPlan, and with this patch we now also check for reductions
directly on VPlan, as well as checking how many load and store
operations remain in the loop.

This should be mostly NFC, but we may compute slightly different
interleave counts, except for some edge cases, e.g. where dead loads
have been removed. This shouldn't happen in practice, and the patch
doesn't cause changes across a large test corpus on AArch64.

Computing the interleave count based on VPlan allows for making better
decisions in presence of VPlan optimizations, for example when
operations on interleave groups are narrowed.

Note that there are a few test changes for tests that were still
checking the legacy cost-model output when it was computed in
selectInterleaveCount.

PR: https://github.com/llvm/llvm-project/pull/149702

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