[AArch64] Allow non-standard INS/DUP encodings

The ARMv8 ARMARM states that for these instructions in A64 state:

  "Unspecified bits in "imm5" are ignored but should be set to zero by an assembler.", (imm4 for INS).

Make the disassembler accept any encoding with these ignored bits set to 1.

llvm-svn: 234896