Replace the PointerLikeTypeTraits::getNumLowBitsAvailable

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@67979 91177308-0d34-0410-b5e6-96231b3b80d8