Improve virtual frame base register allocation heuristics.

  1. Allocate them in the entry block of the function to enable function-wide
     re-use. The instructions to create them should be re-materializable, so
     there shouldn't be additional cost compared to creating them local
     to the basic blocks where they are used.
  2. Collect all of the frame index references for the function and sort them
     by the local offset referenced. Iterate over the sorted list to
     allocate the virtual base registers. This enables creation of base
     registers optimized for positive-offset access of frame references.
     (Note: This may be appropriate to later be a target hook to do the
     sorting in a target appropriate manner. For now it's done here for
     simplicity.)

llvm-svn: 112609