[TableGen] Use less stack in DAGISelMatcherOpt

Refactor a helper function, FactorNodes, to search for a push node in constant space. This resolves a problem in a not-yet-upstreamed backend where a recursive pattern blew the call stack (at a depth of 255) under a debug build of tablegen. No functional change so no new test coverage. The change is minimal to avoid disturbing existing behaviour.

Differential Revision: https://reviews.llvm.org/D29080

llvm-svn: 294230