Fix logic in compareTypesForCast

[AndyAyersMS]

We weren't careful enough with __Canon in some cases, which lead to unsafely
returning MustNot when the cast outcome was not determined at jit time.

Add an extra check, update comments, and add some test cases.

Addresses the failures seen in #1195 (which was reverted).

[AndyAyersMS]

cc @jkotas @EgorBo

No diffs seen in (x64 windows pmi Fx); current callers of checkTypesForCast not impacted.

[EgorBo]

NOTE: compareTypesForCast also always returns MayNot if toHnd is Nullable<> here https://github.com/dotnet/runtime/blob/master/src/coreclr/src/vm/jitinterface.cpp#L4474-L4477

probably makes sense to return MustNot if fromHnd is not a value type.

[jkotas]

Thanks!

[AndyAyersMS]

Formatting failure seems bogus; I did not change the code in the jit, and the patch file produced is empty.

[AndyAyersMS]

x86 libraries "System.Threading.Tasks.Tests Work Item" has failed twice now, leaving no diagnostic output to work with. Fairly sure this is unrelated to this change and it fails intermittently in master too.

Looks like this could be #2271 -- both failures in the above CI runs happen just after the 15 min mark.