[tblgen][disasm] Separate encodings from instructions

Summary:
Separate the concept of an encoding from an instruction. This will enable
the definition of additional encodings for the same instruction which can
be used to support variable length instruction sets in the disassembler
(and potentially assembler but I'm not working towards that right now)
without causing an explosion in the number of Instruction records that
CodeGen then has to pick between.

Reviewers: bogner, charukcs

Reviewed By: bogner

Subscribers: kparzysz, llvm-commits

Differential Revision: https://reviews.llvm.org/D52366

llvm-svn: 349041