[Statepoint Lowering] Consider dead deopt gc values together with other gc values

Currently dead gc value mentioned in the deopt section are not listed in gc section
and so are processed separately.
With this CL all deopt gc values are considered as base pointers and processed in the
same way as other gc values.

The fact that deopt gc pointer is a base pointer was used all the time but
it is explicitly documented here by putting the value in SI.Base.

The idea of the patch comes from Philip Reames.

Reviewers: reames, dantrushin
Reviewed By: reames
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D97554