incr.comp.: `TraitSelect((core::ops::FnOnce, core::ops::FnOnce))` depends on `HirBody` nodes containing closures

[michaelwoerister]

This causes there to be almost no re-use in many cases.

Here's an example of the dependency structure from https://github.com/rust-lang-nursery/rustc-benchmarks/blob/35b45b7f4418b83053f9a46d6b5d2e4b0d253526/regex-0.1.80/makefile#L33:

HirBody(expand::find_cap_ref)
   -> ItemSignature(expand::find_cap_ref::{{closure}})
      -> TraitSelect((core::ops::FnOnce, core::ops::FnOnce)) 
         -> **everything**

cc @nikomatsakis

[nikomatsakis]

will fix shortly