[amdgpu] Handle the case where there is no scavenged register.

- When an unconditional branch is expanded into an indirect branch, if
  there is no scavenged register, an SGPR pair needs spilling to enable
  the destination PC calculation. In addition, before jumping into the
  destination, that clobbered SGPR pair need restoring.
- As SGPR cannot be spilled to or restored from memory directly, the
  spilling/restoring of that SGPR pair reuses the regular SGPR spilling
  support but without spilling it into memory. As that spilling and
  restoring points are fully controlled, we only need to spill that SGPR
  into the temporary VGPR, which needs spilling into its emergency slot.
- The target-specific hook is revised to take additional restore block,
  where the restoring code is filled. After that, the relaxation will
  place that restore block directly before the destination block and
  insert an unconditional branch in any fall-through block into the
  destination block.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D106449