[CodeGen] Use preincrement version of APInt::operator++ instead of postincrement to avoid creating and immediately discarding a temporary APInt.

This is preparation for a clang change to improve the [[nodiscard]] warning to not be ignored on methods that return a class marked [[nodiscard]] that are defined in the class itself. See D32207.

llvm-svn: 300756

clang/lib/CodeGen/CGStmt.cpp

@@ -1166,7 +1166,7 @@ void CodeGenFunction::EmitCaseStmtRange(const CaseStmt &S) {
       if (Rem)
         Rem--;
       SwitchInsn->addCase(Builder.getInt(LHS), CaseDest);
-      LHS++;
+      ++LHS;
     }
     return;
   }