The lowering of the matrix intrinsics is done by the
LowerMatrixIntrinsics pass, which usually runs as part of the
middle-end. The backends do not know how to lower the various matrix
intrinsics however. To avoid llc crashing on IR containing matrix
intrinsics we run a simple/minimal lowering pass, which has no
dependencies besides TTI unconditionally. It will be a no-op for
functions not marked with the may-contain-matrix-intrinsics attribute.