[APInt] Reorder fields to avoid a hole in the middle of the class

Summary:
APInt is currently implemented with an unsigned BitWidth field first and then a uint_64/pointer union. Due to the 64-bit size of the union there is a hole after the bitwidth.

Putting the union first allows the class to be packed. Making it 12 bytes instead of 16 bytes. An APSInt goes from 20 bytes to 16 bytes.

This shows a 4k reduction on the size of the opt binary on my local x86-64 build. So this enables some other improvement to the code as well.

Reviewers: dblaikie, RKSimon, hans, davide

Reviewed By: davide

Subscribers: davide, llvm-commits

Differential Revision: https://reviews.llvm.org/D32001

llvm-svn: 300171