[RewriteStatepointsForGC] Use a worklist algorithm for first part of base pointer algorithm [NFC]

The new code should hopefully be equivalent to the old code; it just uses a worklist to track instructions which need to visited rather than iterating over all instructions visited each time. This should be faster, but the primary benefit is that the purpose should be more clear and the diff of adding another instruction type (forthcoming) much more obvious.

Differential Revision: http://reviews.llvm.org/D11480

llvm-svn: 243071