[X86][NFC] Remove dead code for "_REV" instructions

ADC/SBB with reverse encoding is never emitted by compiler before
encoding optimization, which is called after flag-copy lowering.

This is a partial reland for 8bbf100

Author: KanRobert

llvm/lib/Target/X86/X86FlagsCopyLowering.cpp

@@ -173,7 +173,6 @@ static FlagArithMnemonic getMnemonicFromOpcode(unsigned Opcode) {
 
 #define LLVM_EXPAND_ADC_SBB_INSTR(MNEMONIC)                                    \
   LLVM_EXPAND_INSTR_SIZES(MNEMONIC, rr)                                        \
-  LLVM_EXPAND_INSTR_SIZES(MNEMONIC, rr_REV)                                    \
   LLVM_EXPAND_INSTR_SIZES(MNEMONIC, rm)                                        \
   LLVM_EXPAND_INSTR_SIZES(MNEMONIC, mr)                                        \
   case X86::MNEMONIC##8ri:                                                     \