…to field

In some rare cases (currently only jitstress) morph may miss out in updating a
return of a "struct that can be replaced by its field" with the field, because
we run local assertion prop afterward we check for this update, and assertion
prop, and it may change the local from one that could not be updated into one
that could be.

So do a special run of assertion prop before this update, so that the local we
analyze for possible update is same the one we'll be using in the end.

I tried moving all of this to post-order so local assertion prop only ran
once but that lead to more diffs and some regressions.

Fixes dotnet#74900.