[RISCV] Remove Change field from BlockData in RISCVInsertVSETVLI. NFC

In practice, this field is only used a return value from
computeVLVTYPEChanges. Add a reference parameter to
computeVLVTYPEChanges to return its info.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D158902