[NFC][AArch64] Move AArch64 MSR/MRS into a new decoder namespace

[pratlucas]

Summary

This removes the general forms of the AArch64 MSR and MRS instructions
from the same decoding table that contains many more specific
instructions that supersede them. They're now in a separate decoding
table of their own, called "Fallback", which is only consulted in the
event of the main decoder table failing to produce an answer.

This should avoid decoding conflicts on future specialized instructions
in the MSR space.

Patch written by Simon Tatham.

[ostannard]

LGTM

[pratlucas]

Rebasing.

llvm/lib/Target/AArch64/AArch64InstrFormats.td

let MCOperandPredicate = [{
     return AArch64BTIHint::lookupBTIByEncoding((MCOp.getImm() ^ 32) >> 1) != nullptr;
   }];
 }
 
 class MRSI : RtSystemI<1, (outs GPR64:$Rt), (ins mrs_sysreg_op:$systemreg),
                        "mrs", "\t$Rt, $systemreg"> {
   bits<16> systemreg;
   let Inst{20-5} = systemreg;
+  let DecoderNamespace = "Fallback";
 }
 
 // FIXME: Some of these def NZCV, others don't. Best way to model that?
 // Explicitly modeling each of the system register as a register class
 // would do it, but feels like overkill at this point.
 class MSRI : RtSystemI<0, (outs), (ins msr_sysreg_op:$systemreg, GPR64:$Rt),
                        "msr", "\t$systemreg, $Rt"> {
   bits<16> systemreg;
   let Inst{20-5} = systemreg;
+  let DecoderNamespace = "Fallback";
 }
 
 def SystemPStateFieldWithImm0_15Operand : AsmOperandClass {
   let Name = "SystemPStateFieldWithImm0_15";
   let ParserMethod = "tryParseSysReg";
 }
 def pstatefield4_op : Operand<i32> {
   let ParserMatchClass = SystemPStateFieldWithImm0_15Operand;

llvm/lib/Target/AArch64/Disassembler/AArch64Disassembler.cpp

DecodeStatus AArch64Disassembler::getInstruction(MCInst &MI, uint64_t &Size,
   if (Bytes.size() < 4)
     return Fail;
   Size = 4;
 
   // Encoded as a small-endian 32-bit word in the stream.
   uint32_t Insn =
       (Bytes[3] << 24) | (Bytes[2] << 16) | (Bytes[1] << 8) | (Bytes[0] << 0);
 
-  // Calling the auto-generated decoder function.
-  return decodeInstruction(DecoderTable32, MI, Insn, Address, this, STI);
+  const uint8_t *Tables[] = {DecoderTable32, DecoderTableFallback32};
+
+  for (auto Table : Tables) {
+    DecodeStatus Result =
+        decodeInstruction(Table, MI, Insn, Address, this, STI);
+    if (Result != MCDisassembler::Fail)
+      return Result;
+  }
+
+  return MCDisassembler::Fail;
 }
 
 static MCSymbolizer *
 createAArch64ExternalSymbolizer(const Triple &TT, LLVMOpInfoCallback GetOpInfo,
                                 LLVMSymbolLookupCallback SymbolLookUp,
                                 void *DisInfo, MCContext *Ctx,
                                 std::unique_ptr<MCRelocationInfo> &&RelInfo) {
   return new AArch64ExternalSymbolizer(*Ctx, std::move(RelInfo), GetOpInfo,