Fix some missed opportunities for preInlineUnconditionally
There are two signficant changes here:
Ticket #18815 (closed) showed that we were missing some opportunities for preInlineUnconditionally. The one-line fix is in the code for GHC.Core.Opt.Simplify.Utils.preInlineUnconditionally, which now switches off only for INLINE pragmas. I expanded Note [Stable unfoldings and preInlineUnconditionally] to explain.
When doing this I discovered a way in which preInlineUnconditionally was occasionally /too/ eager. It's all explained in Note [Occurrences in stable unfoldings] in GHC.Core.Opt.OccurAnal, and the one-line change adding markAllMany to occAnalUnfolding.
I also got confused about what NoUserInline meant, so I've renamed it to NoUserInlinePrag, and changed its pretty-printing slightly. That led to soem error messate wibbling, and touches quite a few files, but there is no change in functionality.