Add N2RegVShLFrm and N2RegVShRFrm formats so that the disassembler can easily

dispatch to the appropriate routines to handle the different interpretations of
the shift amount encoded in the imm6 field.  The Vd, Vm fields are interpreted
the same between the two, though.

See, for example, A8.6.367 VQSHL, VQSHLU (immediate) for N2RegVShLFrm format and
A8.6.368 VQSHRN, VQSHRUN for N2RegVShRFrm format.

llvm-svn: 99590