[Fixed Point] Fix C++20 compilation error

Due to how operator== changed after operator<=> landing in C++20
(error: use of overloaded operator '==' is ambiguous (with operand
types 'const llvm::APSInt' and 'const unsigned long')),
CheckIntPartMin/CheckIntPartMax interfaces are now leveraging
APSInt::compareValues.

Differential Revision: https://reviews.llvm.org/D129567