[RS4GC] Introduce intrinsics to get base ptr and offset

There can be a need for some optimizations to get (base, offset)
for any GC pointer. The base can be calculated by generating
needed instructions as it is done by the
RewriteStatepointsForGC::findBasePointer() function. The offset
can be calculated in the same way. Though to not expose the base
calculation and to make the offset calculation as simple as
ptrtoint(derived_ptr) - ptrtoint(base_ptr), which is illegal
outside RS4GC, this patch introduces 2 intrinsics:

 @llvm.experimental.gc.get.pointer.base(%derived_ptr)
 @llvm.experimental.gc.get.pointer.offset(%derived_ptr)

These intrinsics are inlined by RS4GC along with generation of
statepoint sequences.

With these new intrinsics the GC parseable lowering for atomic
memcpy intrinsics (6ec2c5e402a724ba99bce82a9cac7a3006d660f4)
could be implemented as a separate pass.

Reviewed By: reames
Differential Revision: https://reviews.llvm.org/D100445

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