Fix firstEffectfulTerm

[zliu41]

In the Apply case (and similarly the Constr case), firstEffectfulTerm only looks at the function, not the argument, which is both inefficient and incorrect.

An example where it is inefficient:

let !a = error "a" in (\x -> x) a

The inliner is unable to inline a, because firstEffectfulTerm ((\x -> x) a) returns Nothing, but it should.

An example where it is incorrect:

let !a = error "a" in
let !b = (\x -> x) (error "b")
 in a b

The inliner incorrectly inlines a, changing the semantics of the program, because firstEffectfulTerm (let !b = (\x -> x) (error "b") in a b) returns Just a, which is, in turn, because firstEffectfulTerm ((\x -> x) (error "b")) returns Nothing.

This PR fixes it. There are some cost increases because previously it was inlining things that shouldn't have been inlined. That said, we should have a non-conservative mode that allows a to be inlined in the second example.

[michaelpj]

because firstEffectfulTerm (let !b = (\x -> x) (error "b") in a b) returns Just a, which is, in turn, because firstEffectfulTerm ((\x -> x) (error "b")) returns Nothing.

Bad past Michael! Conflating "we know there is no effectful term" with "we don't know whether there is an effectful term"!

[michaelpj]

I'd have been inclined to fold this in as an alternative into FirstEffectfulTerm, but this is also okay.

[michaelpj]

I guess this gives you the alternative instance that you want...

[zliu41]

Exactly.