Auto merge of rust-lang#51411 - nnethercote:process_predicate, r=nikomatsakis

Speed up obligation forest code

Here are the rustc-perf benchmarks that get at least a 1% speedup on one or more of their runs with these patches applied:
```
inflate-check
        avg: -8.7%      min: -12.1%     max: 0.0%
inflate
        avg: -5.9%      min: -8.6%      max: 1.1%
inflate-opt
        avg: -1.5%      min: -2.0%      max: -0.3%
clap-rs-check
        avg: -0.6%      min: -1.9%      max: 0.5%
coercions
        avg: -0.2%?     min: -1.3%?     max: 0.6%?
serde-opt
        avg: -0.6%      min: -1.0%      max: 0.1%
coercions-check
        avg: -0.4%?     min: -1.0%?     max: -0.0%?
```

Author: bors