[MCA] Improved cost computation for loop carried dependencies in the bottleneck analysis.

This patch introduces a cut-off threshold for dependency edge frequences with
the goal of simplifying the critical sequence computation.  This patch also
removes the cost normalization for loop carried dependencies.  We didn't really
need to artificially amplify the cost of loop-carried dependencies since it is
already computed as the integral over time of the delay (in cycle).

In the absence of backend stalls there is no need for computing a critical
sequence. With this patch we early exit from the critical sequence computation
if no bottleneck was reported during the simulation.

llvm-svn: 372337