Improve Dependency analysis when doing multi-node Instruction Selection

Relanding after fixing NodeId Invariant.

Cleanup cycle/validity checks in ISel (IsLegalToFold,
HandleMergeInputChains) and X86 (isFusableLoadOpStore). Now do a full
search for cycles / dependencies pruning the search when topological
property of NodeId allows.

As part of this propogate the NodeId-based cutoffs to narrow
hasPreprocessorHelper searches.

Reviewers: craig.topper, bogner

Subscribers: llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D41293

llvm-svn: 327171