[Reassociate] Skip analysis of dead code to avoid infinite loop.

[Reassociate] Skip analysis of dead code to avoid infinite loop.

Summary:
It was detected that the reassociate pass could enter an inifite
loop when analysing dead code. Simply skipping to analyse basic
blocks that are dead avoids such problems (and as a side effect
we avoid spending time on optimising dead code).

The solution is using the same Reverse Post Order ordering of the
basic blocks when doing the optimisations, as when building the
precalculated rank map. A nice side-effect of this solution is
that we now know that we only try to do optimisations for blocks
with ranked instructions.

Fixes https://llvm.org/bugs/show_bug.cgi?id=30818

Reviewers: llvm-commits, davide, eli.friedman, mehdi_amini

Subscribers: dberlin

Differential Revision: https://reviews.llvm.org/D26154

llvm-svn: 285793