[EarlyCSE] Common gc.relocate calls.

gc.relocate intrinsic is special in that its second and third operands
are not real values, but indices into relocate's parent statepoint list
of GC pointers.
To be CSE'd, they need special handling in `isEqual()` and `getHashCode()`.

Reviewed By: reames
Differential Revision: https://reviews.llvm.org/D80445