Fix for StackOverflow in SsaBuilder::TopologicalSort.

Fix for StackOverflow in SsaBuilder::TopologicalSort.

SsaBuilder::TopologicalSort allocated an array of bools on the stack.
In one of internal cases the array had 126,758 elements, which caused a
StackOverflow. This change switches SsaBuilder::TopologicalSort from
stack-allocated array of bools to BitVec.

TopologicalSortHelper used to be recursive. It was changed to use an iterative
algorithm in a previous change. I'm inlining TopologicalSortHelper into
TopologicalSort now.

I verified no measurable negative throughput impact from this change
and no asm diffs in SuperPMI.