[APInt] Add ashrInPlace method and rewrite ashr to make a copy and then call ashrInPlace.

This patch adds an in place version of ashr to match lshr and shl which were recently added.

I've tried to make this similar to the lshr code with additions to handle the sign extension. I've also tried to do this with less if checks than the current ashr code by sign extending the original result to a word boundary before doing any of the shifting. This removes a lot of the complexity of determining where to fill in sign bits after the shifting.

Differential Revision: https://reviews.llvm.org/D32415

llvm-svn: 301198