[MIR] Fix vreg flag vector memory leak (#112479)

A fix-it patch for dbfca24 #110228.

No need for a container. This allows 8 flags for a register.

The virtual register flags vector had a memory leak because the vector's
memory is not freed.
The `BumpPtrAllocator` handles the deallocation and missed calling the
`std::vector<uint8_t> Flags` destructor.

Author: optimisan

llvm/include/llvm/CodeGen/MIRParser/MIParser.h

@@ -45,7 +45,7 @@ struct VRegInfo {
   } D;
   Register VReg;
   Register PreferredReg;
-  std::vector<uint8_t> Flags;
+  uint8_t Flags = 0;
 };
 
 using Name2RegClassMap = StringMap<const TargetRegisterClass *>;

llvm/lib/CodeGen/MIRParser/MIRParser.cpp

@@ -703,7 +703,7 @@ bool MIRParserImpl::parseRegisterInfo(PerFunctionMIParsingState &PFS,
         return error(FlagStringValue.SourceRange.Start,
                      Twine("use of undefined register flag '") +
                          FlagStringValue.Value + "'");
-      Info.Flags.push_back(FlagValue);
+      Info.Flags |= FlagValue;
     }
     RegInfo.noteNewVirtualRegister(Info.VReg);
   }