[TypePromotion] Avoid some unnecessary truncs

[samparker]

Summary

Check for legal zext 'sinks' before inserting a trunc.

I think this resolves some/most/all of the issue in D111237. I won't have the time to fix this if it's broken though.

edit: I setup a stage 2 on my mac and this looks fine enough.

[dmgreen]

Sorry for the delay. This doesn't fix the original case unfortunately, as that version doesn't start with an icmp. The test case is a little different with the loads feeding the loop condition.
But sounds like it should be an improvement, so LGTM (I would suggest committing after the branch ;) )