[mips][msa] Implemented build_vector using ldi, fill, and custom SelectionDAG nodes (VSPLAT and VSPLATD)

Note: There's a later patch on my branch that re-implements this to select
build_vector without the custom SelectionDAG nodes. The future patch avoids
the constant-folding problems stemming from the custom node (i.e. it doesn't
need to re-implement all the DAG combines related to BUILD_VECTOR).

Changes to MIPS specific SelectionDAG nodes:
* Added VSPLAT
    This is a special case of BUILD_VECTOR that covers the case the
    BUILD_VECTOR is a splat operation.
* Added VSPLATD
    This is a special case of VSPLAT that handles the cases when v2i64 is legal