[IR] Deleted some stale commented out code.
[IR] Fix longstanding bug setting up splats in call args.

* Discovered ArgumentError failures in MRI test runs -- found out
  that CallBase was unsplatting a splat which shouldn't have been.
  Try "p [*1..2], 3" before/after.

* This patch fixes it and as a bonus, cleans out an ugly hack that
  was present in buildCallArgs in IRBuilder.

* In the process renamed 'containsSplat' in CallBase to
  'containsArgSplat' -- check if there is a splat in the arg list
  that should be unsplatted while setting up call args.

* 2 more MRI tests now green.