Replace the PointerLikeTypeTraits::getNumLowBitsAvailable

function with a new NumLowBitsAvailable enum, which makes the
value available as an integer constant expression.

Add PointerLikeTypeTraits specializations for Instruction* and
Use** since they are only guaranteed 4-byte aligned.

Enhance PointerIntPair to know about (and enforce) the alignment
specified by PointerLikeTypeTraits.  This should allow things 
like PointerIntPair<PointerIntPair<void*, 1,bool>, 1, bool> 
because the inner one knows that 2 low bits are free.

llvm-svn: 67979