Float unboxed expressions by boxing
This patch makes GHC's floating more robust, by allowing it to float unboxed expressions of at least some common types. See Note [Floating MFEs of unlifted type] in SetLevels. This was all provoked by Trac #12603 In working this through I also made a number of other corner-case changes in SetLevels: * Previously we inconsistently use exprIsBottom (which checks for bottom) instead of exprBotStrictness_maybe (which checks for bottoming functions). As well as being inconsistent it was simply less good. See Note [Bottoming floats] * I fixed a case where were were unprofitably floating an expression because we thought it escaped a value lambda (see Note [Escaping a value lambda]). The relevant code is float_me = (dest_lvl `ltMajLvl` (le_ctxt_lvl env) && not float_is_lam) -- NEW * I made lvlFloatRhs work properly in the case where abs_vars is non-empty. It wasn't wrong before, but it did some stupid extra floating.
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