[Matrix] Preserve volatile when loading loads/stores.

Currently the matrix lowering turns volatile loads/stores into
non-volatile ones. This patch updates the lowering to preserve the
volatile bit.

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

Reviewed By: anemet

Differential Revision: https://reviews.llvm.org/D81498