TempRValueOptPass: some small cosmetic changes

1. WitnessMethodInst doesn't need to be handled as use because it's operand is a typedependent operand, which we exclude anyway when looking at the uses.
2. Replace a list of handled instructions with a default value in the switch. It's easy to forget to add an instruction here. So it's safer to have a default behavior.

Author: eeckstein

lib/SILOptimizer/Transforms/CopyForwarding.cpp

@@ -1714,12 +1714,10 @@ bool TempRValueOptPass::collectLoads(
 
   case SILInstructionKind::LoadInst:
   case SILInstructionKind::LoadBorrowInst:
-  case SILInstructionKind::WitnessMethodInst: {
     // Loads are the end of the data flow chain. The users of the load can't
     // access the temporary storage.
     loadInsts.insert(user);
     return true;
-  }
 
   case SILInstructionKind::CopyAddrInst: {
     // copy_addr which read from the temporary are like loads.
@@ -1834,17 +1832,9 @@ bool TempRValueOptPass::tryOptimizeCopyIntoTemp(CopyAddrInst *copyInst) {
       use->set(copyInst->getSrc());
       break;
     }
-    case SILInstructionKind::StructElementAddrInst:
-    case SILInstructionKind::TupleElementAddrInst:
-    case SILInstructionKind::LoadInst:
-    case SILInstructionKind::LoadBorrowInst:
-    case SILInstructionKind::ApplyInst:
-    case SILInstructionKind::OpenExistentialAddrInst:
+    default:
       use->set(copyInst->getSrc());
       break;
-
-    default:
-      llvm_unreachable("unhandled instruction");
     }
   }
   tempObj->eraseFromParent();