Optimize CDG construction
Optimize CDG construction

Adds a guard to reduce the number of postdominance queries, which
ended up being fairly expensive for libraries with huge basic
blocks (e.g., pixman).  The intuition is that only edges between basic
blocks really matter here, so ignore the rest cheaply and avoid
redundant postdom queries.

wangxiayang/llvm-analysis,travitch/llvm-analysis,wangxiayang/llvm-analysis,travitch/llvm-analysis