Assume at least one evaluation for nested SubDemands (#21081)

See the new Note [SubDemand denotes at least one evaluation].

A demand n :* sd on a let binder x=e now means

"x was evaluated n times and in any program trace it is evaluated, e is evaluated deeply in sub-demand sd."

The "any time it is evaluated" premise is what this patch adds. As a result, we get better nested strictness. For example (T21081)

f :: (Bool, Bool) -> (Bool, Bool)
f pr = (case pr of (a,b) -> a /= b, True)
-- before: <MP(L,L)>
-- after:  <MP(SL,SL)>

g :: Int -> (Bool, Bool)
g x = let y = let z = odd x in (z,z) in f y

The change in demand signature "before" to "after" allows us to case-bind z here.

Similarly good things happen for the sd in call sub-demands Cn(sd), which allows for more eta-reduction (which is only sound with -fno-pedantic-bottoms, albeit).

We also fix #21085 (closed), a surprising inconsistency with Poly to Call sub-demand expansion.

In an attempt to fix a regression caused by less inlining due to eta-reduction in T15426, I eta-expanded the definition of elemIndex and elemIndices, thus fixing #21345 (closed) on the go.

The main point of this patch is that it fixes #21081 (closed) and #21133 (closed).

Annoyingly, I discovered that more precise demand signatures for join points can transform a program into a lazier program if that join point gets floated to the top-level, see #21392. There is no simple fix at the moment, but !5349 might. Thus, we accept a ~5% regression in MultiLayerModulesTH_OneShot, where #21392 bites us in addListToUniqDSet. T21392 reliably reproduces the issue.