Bug 1019304 - Part 2: Overhaul PJS bailout mechanism to be like the normal bailout mechanism. (r=nmatsakis)

- Remove ad-hoc IR tracing and AbortPar machinery.
- PJS bailouts use a different handler that unwinds the entire ForkJoin
  stack, but otherwise shares the same code as sequential bailouts.
- Each thread's stack is snapshotted as a RematerializedFrame vector.
Bug 1024756 - Remove unnecessary saving of lengthReg in NewDenseArrayPar codegen.