[APInt] Move the 'return *this' from the slow cases of assignment operators inline...

[APInt] Move the 'return *this' from the slow cases of assignment operators inline. We should let the compiler see that the fast/slow cases both return *this.

I don't think we chain assignments together very often so this shouldn't matter much.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@300715 91177308-0d34-0410-b5e6-96231b3b80d8