[Matrix] Hoist load/store generation logic, add helpers for tiled access.

This patch slightly generalizes the code to emit loads and stores of a
matrix and adds helpers to load/store a tile of a larger matrix.

This will be used in a follow-up patch introducing initial tiling.

Reviewers: anemet, Gerolf, hfinkel, andrew.w.kaylor, LuoYuanke

Reviewed By: anemet

Differential Revision: https://reviews.llvm.org/D75564