Eliminate special-case break/nonlocal return handling for lambdas

* Lambdas need to trap breaks/nonlocal-returns and terminate them.
  However, whether a block becomes a lambda or not is determined
  at runtime. So, only blocks that becomes lambdas need a try/catch
  to trap these breaks/nonlocal-returns. So far, in the interp,
  we used to tackle these by adding the try/catch at runtime on
  first execution.

* This patch eliminates that runtime check and unconditionally
  adds the try/catch for all blocks (lambdas or not). Did this by
  uncommenting the unconditional handling that was there in
  IRBuilder.

  To make sure everything continued to work fine, fixed non-local
  return handling in IRReturnHelpers to ignore them if the
  blockType is not null (methods) or a lambda. This ensures
  that independent of what exists on the scope-stack, we only test
  for a matching non-local returns in lambdas or method scopes.

* Had to fix up addition of global-ensure-blocks in store-local-var
  analysis to only add a GEB if one didn't already exist.

* Eliminated all lambda-specific tests while preparing instructions
  for interpretation/JITting.

* Unrelated:
  - Fixed toStringOutput of BacktickInstr