[LAA] Precommit some extra tests for forked pointers

  * Converted tests to use opaque pointers
  * Added suggested test for inbounds GEP
  * Added a test for forks on both the base and offset terms of a GEP
  * Added a test for a select of a select
  * Added a test for a GEP with >2 operands
  * Added a test for vector GEPs