[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.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300756 91177308-0d34-0410-b5e6-96231b3b80d8