-----------------------------------------------
	Fix a terrible and long-standing strictness bug
	-----------------------------------------------

		MERGE TO STABLE

Simplify.simplifyArg was floating out lets from a lazy argument.
Not only is this unprofitable, but it can actually be wrong it
the floated let has a strict-demand flag on it.  (Simplify.simplLazyBind
goes to some trouble to check for this case.)

The situation is this

	lazy_fn (let v = <expensive> in str_fn v v)

Here, the strictness analyser may put a 'demanded' flag on
v, and if is is *then* floated we'll get

	let v* = <expensive>
	in
	lazy_fn (str_fn v v)

and then <expensive> will always be evaluated.

This bug has been in the compiler at least a year (since Sept 01), but
in fact it's quite hard to make a test case for it, because the same
bug meant that the let was floated out *before* strictness analysis
giving

	let v = <expensive>
	in
	lazy_fn (str_fn v v)

and now v won't get a strict-demand flag.  So it's only if the let
shows up (by inlining) after strictness analysis but not before.