Decompose GVN::processNonLocalLoad() (about 400 LOC) into smaller helper functions. No function change.

This function consists of following steps:
   1. Collect dependent memory accesses.
   2. Analyze availability.
   3. Perform fully redundancy elimination, or
   4. Perform PRE, depending on the availability

 Step 2, 3 and 4 are now moved to three helper routines.