[APInt] Simplify the zext and sext methods

This replaces a hand written copy loop with a call to memcpy for both zext and sext.

For sext, it replaces multiple if/else blocks propagating sign information forward. Now we just do a copy, a sign extension on the last copied word, a memset, and clearUnusedBits.

Differential Revision: https://reviews.llvm.org/D32417

llvm-svn: 301201