Hoisting of loop-invariant subexpressions

[lukaszcz]

• Closes Lift complex expression that don't use the lambda-bound variables out of the lambdas #3028
• Detects fully-applied expressions of the form

f a (\{ x1 .. xk := body }) a'

where f is a recursive function which does not modify its function argument (the one for which a lambda is provided), i.e., passes it to recursive invocations unchanged (it's the second argument in the above example). A function f satisfying this requirement implements some kind of loop with the lambda-expression usually being the loop body / action.

Let e.g. body = g x1 .. xk b1 b2 where b1, b2 do not contain any variables bound in the lambda (x1,..,xk). If b1, b2 are non-trivial (require computation), then these loop-invariant subexpressions are hoisted outside of the loop, obtaining:

let 
  y1 := b1;
  y2 := b2;
in
f a (\{ x1 .. xk := g x1 .. xk y1 y2) a'

• Adds a volatility info in Core which prevents the lets created by loop-invariant expression hoisting from being folded back.
• Removes potential duplication in the specialization optimization.
• Adds another optimization phase before lambda-lifting where hoisting of loop-invariant subexpressions is performed. This allows to reduce the number of iterations in the main optimization phase after lambda-lifting.