[llvm] Reapply "[MC][TableGen] Expand Opcode field of MCInstrDesc" (#180321) (PR #180954)

[Matt Arsenault via llvm-commits]

arsenm wrote:

Is there a real reason to use the encoded instruction in those patterns? 

Long term I would like to remove the custom system of pseudo + set of encoded versions of instructions which would slash the number of opcode definitions. The current system is a clumsier replacement for EncodingByHwMode 

https://github.com/llvm/llvm-project/pull/180954