[IR] Strip no-longer-needed ensure-map and related state

* git 27442fc0 fixed some bugs in the interpreter that effectively
  used the rescue map for handling both rescue and ensures which
  left the ensure map unused.

* This patch got rid of the ensure map and all related state from
  the IR builder, instructions, CFG, and exception regions.

* Verified that there is no change in spec test results especially
  since the CFG removed exception edges to ensure-block, which could
  potentially have affected accuracy of the opt. passes.