Kill unnecessary VLMap updates.

 - A branch instruction doesn't define a local variable, so there's no
   need to update the map.

 - Similarly only update the VLMap for call instructions if there is a
   return value.

No functional change.

Author: vext01

llvm/lib/YkIR/YkIRWriter.cpp

@@ -431,7 +431,9 @@ class YkIRWriter {
       serialiseOperand(I, VLMap, I->getOperand(OI));
     }
 
-    VLMap[I] = {BBIdx, InstIdx};
+    if (!I->getType()->isVoidTy()) {
+      VLMap[I] = {BBIdx, InstIdx};
+    }
     InstIdx++;
   }
 
@@ -447,8 +449,6 @@ class YkIRWriter {
       // num_operands:
       // We don't serialise any operands, because traces will guide us.
       OutStreamer.emitInt32(0);
-
-      VLMap[I] = {BBIdx, InstIdx};
     } else {
       // type_index:
       OutStreamer.emitSizeT(typeIndex(I->getType()));