Improve worker/wrapper when no w/w is needed
f x = x
Strictnesss analysis does not lead to a w/w split, so the obvious thing is to leave it 100% alone. But actually, because the RHS is small, we ended up adding a StableUnfolding for it.
There is some reason to do this if we choose not do to w/w on the grounds that the function is small. See
Note [Don't w/w inline small non-loop-breaker things]
But there is no reason if we would not have done w/w anyway.
The solution is to move the conditional to later. Easy.
In investigating this I also discovered an outright bug; namely that the worker/wrapper phase would overwrite
InlineStable, which is utterly wrong. That in turn meant that some default methods (slightly oddly marked InlineCompulsory -- #17914) were getting their InlineCompulsory squashed.