[mlir][Linalg] Add a hoistRedundantVectorTransfers helper function

This revision adds a helper function to hoist vector.transfer_read /
vector.transfer_write pairs out of immediately enclosing scf::ForOp
iteratively, if the following conditions are true:
   1. The 2 ops access the same memref with the same indices.
   2. All operands are invariant under the enclosing scf::ForOp.
   3. No uses of the memref either dominate the transfer_read or are
   dominated by the transfer_write (i.e. no aliasing between the write and
   the read across the loop)

To improve hoisting opportunities, call the `moveLoopInvariantCode` helper
function on the candidate loop above which to hoist. Hoisting the transfers
results in scf::ForOp yielding the value that originally transited through
memory.

This revision additionally exposes `moveLoopInvariantCode` as a helper in
LoopUtils.h and updates SliceAnalysis to support return scf::For values and
allow hoisting across multiple scf::ForOps.

Differential Revision: https://reviews.llvm.org/D81199