[RISCV] Decode vtype with reserved fields to raw immediate

This patch fixes a crash when doing "llvm-objdump -D --mattr=+experimental-v"
against an object file which happens to keep a word that can be decoded to
VSETVLI & VSETIVLI with reserved vlmul[2:0]=4. All vtype values with
reserved fields (vlmul[2:0]=4, vsew[2:0]=0b1xx, non-zero bits 8/9/10) are
printed to raw immediate.

Reviewed By: jhenderson, jrtc27, craig.topper

Differential Revision: https://reviews.llvm.org/D114581