[ValueTypes] Rename MVT::getVectorNumElements() to...

[ValueTypes] Rename MVT::getVectorNumElements() to MVT::getVectorMinNumElements(). Fix some misuses of getVectorNumElements()

getVectorNumElements() returns a value for scalable vectors
without any warning so it is effectively getVectorMinNumElements().
By renaming it and making getVectorNumElements() forward to
it, we can insert a check for scalable vectors into getVectorNumElements()
similar to EVT. I didn't do that in this patch because there are still more
fixes needed, but I was able to temporarily do it and passed the RISCV
lit tests with these changes.

The changes to isPow2VectorType and getPow2VectorType are copied from EVT.

The change to TypeInfer::EnforceSameNumElts reduces the size of AArch64's isel table.
We're now considering SameNumElts to require the scalable property to match which
removes some unneeded type checks.

This was motivated by the bug I fixed yesterday in 80b95108

Reviewed By: frasercrmck, sdesmalen

Differential Revision: https://reviews.llvm.org/D102262