[CodeGen] Use preincrement version of APInt::operator++ instead of...

[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