Revert an earlier change to SIGN_EXTEND_INREG for vectors. The VTSDNode

really does need to be a vector type, because
TargetLowering::getOperationAction for SIGN_EXTEND_INREG uses that type,
and it needs to be able to distinguish between vectors and scalars.

Also, fix some more issues with legalization of vector casts.

llvm-svn: 93043