Use RValue refs in APInt add/sub methods.

This adds versions of operator + and - which are optimized for the LHS/RHS of the
operator being RValue's.  When an RValue is available, we can use its storage space
instead of allocating new space.

On code such as ConstantRange which makes heavy use of APInt's over 64-bits in size,
this results in significant numbers of saved allocations.

Thanks to David Blaikie for all the review and most of the code here.

llvm-svn: 276470