[Statepoint Lowering] Fix the crash with gc.relocate in a separate block

If it was decided to relocate derived pointer using the spill its value is
not exported in general case.
When gc.relocate is located in an another block than a statepoint we cannot
get SD for derived value but for spill case it is not required at all.
However implementation of gc.relocate lowering unconditionally request SD value
causing the assert triggering.

The CL fixes this by handling spill case earlier than SD is really required.

Reviewers: reames, dantrushin
Reviewed By: dantrushin
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D98324