More inlining changes from SimonPJ & me: The Plan is to avoid relying
on knowledge of OneOcc occurrences after postInlineUnconditionally, so
we now attempt to make use of OneOcc as far as possible in in
pre/postInlineUnconditionally rather than the call site.  Plenty of
comments in the code with more details.

This change almost always improves performance on nofib; we have one
program that goes slower, which we'll investigate in due course.