Mips: Remove immarg from copy and insert intrinsics

These intrinsics do in fact work with non-constant index arguments.

These are lowered to either the generic
ISD::INSERT_VECTOR_ELT/ISD::EXTRACT_VECTOR_ELT, or to
VEXTRACT_SEXT_ELT. The handling of these all accept variable
indexes. Turning these into generic instructions which do allow
variables introduces complications in a future change to immarg
handling.

Since these just turn into generic instructions, these are kind of
pointless and should probably just be autoupgraded to
extractelement/insertelement.

llvm-svn: 366328