Optimize DAGCombiner's worklist processing. Previously it started

its work by putting all nodes in the worklist, requiring a big
dynamic allocation. Now, DAGCombiner just iterates over the AllNodes
list and maintains a worklist for nodes that are newly created or
need to be revisited. This allows the worklist to stay small in most
cases, so it can be a SmallVector.

This has the side effect of making DAGCombine not miss a folding
opportunity in alloca-align-rounding.ll.

llvm-svn: 55498