Drop the app invariant

let’s see what happens… fixes #20554.

Enforce Let-invariant in `simplLam`; testcase

Enforce the let-invariant in SimpleOpt

Use mightBeUnliftedType, not isUnliftedType

Like this? But I still get these errors

Implement some of Simon’s suggestions

This seems to be the right SimplEnv

In simplNonRecE, create StrictBind when let-invariant is not satisfied

because `simplLazyBind` assumes that invariant.

prepareRHS must not call makeTrivial when a case would be needed

Try to avoid exprType a bit (does not help, though)

Avoid exponential recursion in exprOkForSpeculation

More use of mightBeUnliftedType

else we get a panic

case-of-known-constructor: Float unlifted arguments

Teach DmdAnal about unlifted arguments (H/T Sebastian G)

Update test output (mostly strictness signatures)