Add NVTBLFrm to represent A8.6.406 VTBL, VTBX Vector Table Lookup Instructions.

Add NVTBLFrm to represent A8.6.406 VTBL, VTBX Vector Table Lookup Instructions.
These instructions use byte index in a control vector (M:Vm) to lookup byte
values in a table and generate a new vector (D:Vd).  The table is specified via
a list of vectors, which can be:

{Dn}
{Dn D<n+1>}
{Dn D<n+1> D<n+2>}
{Dn D<n+1> D<n+2> D<n+3>}

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@99789 91177308-0d34-0410-b5e6-96231b3b80d8