Introduce new SelectionDAG node opcodes VEXTRACT_SUBVECTOR and

VCONCAT_VECTORS. Use these for CopyToReg and CopyFromReg legalizing in
the case that the full register is to be split into subvectors instead
of scalars. This replaces uses of VBIT_CONVERT to present values as
vector-of-vector types in order to make whole subvectors accessible via
BUILD_VECTOR and EXTRACT_VECTOR_ELT.

This is in preparation for adding extended ValueType values, where
having vector-of-vector types is undesirable.

llvm-svn: 37569