GlobalISel: Make known bits/alignment API more consistent

Just computing the alignment makes sense without caring about the
general known bits, such as for non-integral pointers. Separate the
two and start calling into the TargetLowering hooks for frame indexes.

Start calling the TargetLowering implementation for FrameIndexes,
which improves the AMDGPU matching for stack addressing modes. Also
introduce a new hook for returning known alignment of target
instructions. For AMDGPU, it would be useful to report the known
alignment implied by certain intrinsic calls.

Also stop using MaybeAlign.