SelectionDAG: Remove #if NDEBUG from check for a post-isel hook

The InstrEmitter will skip the check of MI.hasPostISelHook()
before calling AdjustInstrPostInstrSelection() when NDEBUG
is not defined.

This was added in r140228, and I'm not sure if it is intentional or not,
but it is a likely source for bugs, because it means with
Release+Asserts builds you can forget to set the hasPostISelHook
flag on TableGen definitions and AdjustInstrPostInstrSelection() will
still be called.
ARM: Remove unneeded check for MI->hasPostISelHook()