[AMDGPU][MC] Disassembler warning for v_cmpx instructions

For GFX10+ the destnation reg of v_cmpx instructions is
implicitly EXEC, which is encoded as 0x7E. However, the
disassembler does not check this field, thus allowing any
value. With this patch, if the field is not 0x7E a warning
is issued.

Author: jwanggit86

llvm/lib/Target/AMDGPU/Disassembler/AMDGPUDisassembler.cpp

@@ -663,6 +663,8 @@ DecodeStatus AMDGPUDisassembler::getInstruction(MCInst &MI, uint64_t &Size,
     return MCDisassembler::Fail;
   } while (false);
 
+  DecodeStatus Status = MCDisassembler::Success;
+
   if (MCII->get(MI.getOpcode()).TSFlags & SIInstrFlags::DPP) {
     if (isMacDPP(MI))
       convertMacDPPInst(MI);
@@ -801,8 +803,16 @@ DecodeStatus AMDGPUDisassembler::getInstruction(MCInst &MI, uint64_t &Size,
   if (ImmLitIdx != -1 && !IsSOPK)
     convertFMAanyK(MI, ImmLitIdx);
 
+  if (isGFX10Plus() &&
+      (MCII->get(MI.getOpcode()).TSFlags & SIInstrFlags::VOP3) &&
+      MCII->get(MI.getOpcode()).hasImplicitDefOfPhysReg(AMDGPU::EXEC)) {
+    auto ExecEncoding = MRI.getEncodingValue(AMDGPU::EXEC_LO);
+    if (Bytes_[0] != ExecEncoding)
+      Status = MCDisassembler::SoftFail;
+  }
+
   Size = MaxInstBytesNum - Bytes.size();
-  return MCDisassembler::Success;
+  return Status;
 }
 
 void AMDGPUDisassembler::convertEXPInst(MCInst &MI) const {