Rollup merge of #51399 - ngg:nll-performance, r=nikomatsakis

NLL performance boost

This makes compilation of the [inflate](https://github.com/rust-lang-nursery/rustc-perf/tree/master/collector/benchmarks/inflate) benchmark compile 2 times faster on my computer when NLL is enabled.
This does not fix the #51377 perfectly, it's still 4-5 times slower than without NLL, but it's a start.