[IR] Avoid redundant map lookup [NFC]

Use the iterator returned by MapVector::insert to update
the value in the map, instead of a second redundant map lookup.

Author: jasilvanus

llvm/lib/IR/Operator.cpp

@@ -229,8 +229,8 @@ bool GEPOperator::collectOffset(
     // Insert an initial offset of 0 for V iff none exists already, then
     // increment the offset by IndexedSize.
     if (!IndexedSize.isZero()) {
-      VariableOffsets.insert({V, APInt(BitWidth, 0)});
-      VariableOffsets[V] += IndexedSize;
+      auto *It = VariableOffsets.insert({V, APInt(BitWidth, 0)}).first;
+      It->second += IndexedSize;
     }
   }
   return true;