[mlir][sparse] hoist loop invariant tensor loads in sparse compiler

[mlir][sparse] hoist loop invariant tensor loads in sparse compiler

After bufferization, the backend has much more trouble hoisting loop invariant
loads from the loops generated by the sparse compiler. Therefore, this is done
during sparse code generation. Note that we don't bother hoisting derived
invariant expressions on SSA values, since the backend does that very well.

Still TBD: scalarize reductions to avoid load-add-store cycles

Reviewed By: penpornk

Differential Revision: https://reviews.llvm.org/D92534