[mlir][LLVM] Add `BitcastStores` type-consistency pattern

Current patterns attempt to immediately create type-consistent stores by bitcasting the value to the type-hint leading to needless code duplication.

This patch extracts that case into its own pattern, allowing other patterns to create type-inconsistent stores and have subsequent pattern applications turn it into a type-consistent store.

Differential Revision: https://reviews.llvm.org/D154587