ValueTracking: Handle !absolute_symbol in computeKnownBits

Use a unit test since I don't see any existing uses try to make use of
the high bits of a pointer.

This will also assert if the metadata type doesn't match the pointer
width, but I consider that a defect in the verifier and shouldn't be
handled.

AMDGPU allocates LDS globals by assigning !absolute_symbol with the
final fixed address. Tracking the high bits are 0 may help with
addressing mode matching.