[APInt] Remove self move check from move assignment operator

This was added to work around a bug in MSVC 2013's implementation of stable_sort. That bug has been fixed as of MSVC 2015 so we shouldn't need this anymore.

Technically the current implementation has undefined behavior because we only protect the deleting of the pVal array with the self move check. There is still a memcpy of that.VAL to VAL that isn't protected. In the case of self move those are the same local and memcpy is undefined for src and dst overlapping.

This reduces the size of the opt binary on my local x86-64 build by about 4k.

Differential Revision: https://reviews.llvm.org/D32116

llvm-svn: 300477