Note [Inline unsafeCoerce]

We really want to inline unsafeCoerce, even when applied to boring
arguments.  It doesn't look as if its RHS is smaller than the call
   unsafeCoerce x = case unsafeEqualityProof @a @b of UnsafeRefl -> x
but that case is discarded in CoreToStg -- see Note [Implementing unsafeCoerce]
in base:Unsafe.Coerce.

Moreover, if we /don't/ inline it, we may be left with
          f (unsafeCoerce x)
which will build a thunk -- bad, bad, bad.

Conclusion: we really want inlineBoringOk to be True of the RHS of
unsafeCoerce. And it really is, because we regard
  case unsafeEqualityProof @a @b of UnsafeRefl -> rhs
as trivial iff rhs is. This is (U4) in Note [Implementing unsafeCoerce].