Prep: Refactor for more flexible FloatingBinds

Discussion started in !10088 (comment 499584). Today, we have

data FloatingBind
  = FloatLet CoreBind    -- Rhs of bindings are CpeRhss
                         -- They are always of lifted type;
                         -- unlifted ones are done with FloatCase

 | FloatCase
      CpeBody         -- Always ok-for-speculation
      Id              -- Case binder
      AltCon [Var]    -- Single alternative
      Bool            -- Ok-for-speculation; False of a strict,
                      -- but lifted binding

 -- | See Note [Floating Ticks in CorePrep]
 | FloatTick CoreTickish

Issues:

1. We never use FloatCase with something different than a DEFAULT AltCon, so it really is just a strict Let.
2. We cache for a FloatCase whether it's ok-for-spec-eval. If it is, we may float it freely out of lazy contexts while retaining the strict binding semantics. But if the binding is not ok-for-spec-eval (e.g., a regular case binding), we may not float it out of lazy contexts, unless we change it into a lazy float (the benefit of which is far from clear and not the subject of this issue).
3. An ok-for-spec-eval FloatCase is not much different to a regular lazy FloatLet, yet they are represented completely differently.

In a call, Simon and I came up with the following idea:

data FloatingBind
  = Float
      CoreBind  
      RhsInfo

 -- | See Note [Floating Ticks in CorePrep]
 | FloatTick CoreTickish
 
 
data RhsInfo
  = NormalForm    -- Literal or saturated constructor application with trivial arguments
  | OkForSpec     -- Lifted but ok-for-spec
  | NotOkForSpec  -- Lifted and perhaps not ok-for-spec
  | StrictlyUsed  -- Used strictly or is of unlifted type, might be ok-for-spec or not. Don't float out of lazy binds

Much simpler and more expressive.