Reapply 267210 with fix for PR27490

Original Commit Message
Extend load/store type canonicalization to handle unordered operations

Extend the type canonicalization logic to work for unordered atomic loads and stores.  Note that while this change itself is fairly simple and low risk, there's a reasonable chance this will expose problems in the backends by suddenly generating IR they wouldn't have seen before.  Anything of this nature will be an existing bug in the backend (you could write an atomic float load), but this will definitely change the frequency with which such cases are encountered.  If you see problems, feel free to revert this change, but please make sure you collect a test case. 

Note that the concern about lowering is now much less likely.  PR27490 proved that we already *were* mucking with the types of ordered atomics and volatiles.  As a result, this change doesn't introduce as much new behavior as originally thought.

llvm-svn: 268809