a DenseMap.  Doing this required being aware of subtle iterator
invalidation issues, but it provides a big speedup.  In a 
release-asserts build, this sped up optimizing 403.gcc from
1.34s -> 0.79s (IPSCCP) and 1.11s -> 0.44s (SCCP).

This commit also conflates in a bunch of general cleanups, sorry.

llvm-svn: 85788