Improve FloatOut and SpecConstrwip/T21386

This patch addresses a relatively obscure situation that arose
when chasing perf regressions in !7847, which itself is fixing

It does two things:

* SpecConstr can specialise on ($df d1 d2) dictionary arguments
* FloatOut no longer checks argument strictness

See Note [Specialising on dictionaries] in GHC.Core.Opt.SpecConstr.

A test case is difficult to construct, but it makes a big difference
in nofib/real/eff/VSM, at least when we have the patch for #21286
installed. (The latter stops worker/wrapper for dictionary arguments).

There is a spectacular, but slightly illusory, improvement in
runtime perf on T15426.  I have documented the specifics in
T15426 itself.

Metric Decrease:
    T15426

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