[APInt] Move isMask and isShiftedMask out of APIntOps and into the APInt...

[APInt] Move isMask and isShiftedMask out of APIntOps and into the APInt class. Implement them without memory allocation for multiword

This moves the isMask and isShiftedMask functions to be class methods. They now use the MathExtras.h function for single word size and leading/trailing zeros/ones or countPopulation for the multiword size. The previous implementation made multiple temorary memory allocations to do the bitwise arithmetic operations to match the MathExtras.h implementation.

Differential Revision: https://reviews.llvm.org/D31565

llvm-svn: 299362