Auto merge of #2328 - RalfJung:perf, r=RalfJung
Auto merge of #2328 - RalfJung:perf, r=RalfJung

move checking ptr tracking on item pop into cold helper function

Before:
```
Benchmark 1: cargo miri run --manifest-path bench-cargo-miri/serde1/Cargo.toml
  Time (mean ± σ):      6.729 s ±  0.050 s    [User: 6.608 s, System: 0.124 s]
  Range (min … max):    6.665 s …  6.799 s    5 runs

Benchmark 2: cargo miri run --manifest-path bench-cargo-miri/unicode/Cargo.toml
  Time (mean ± σ):     20.923 s ±  0.271 s    [User: 20.386 s, System: 0.537 s]
  Range (min … max):   20.580 s … 21.165 s    5 runs
```
After:
```
Benchmark 1: cargo miri run --manifest-path bench-cargo-miri/serde1/Cargo.toml
  Time (mean ± σ):      6.562 s ±  0.023 s    [User: 6.430 s, System: 0.135 s]
  Range (min … max):    6.544 s …  6.594 s    5 runs

Benchmark 2: cargo miri run --manifest-path bench-cargo-miri/unicode/Cargo.toml
  Time (mean ± σ):     20.375 s ±  0.228 s    [User: 19.964 s, System: 0.413 s]
  Range (min … max):   20.201 s … 20.736 s    5 runs
```
Nothing major, but we'll take it I guess. :shrug:

Fixes https://github.com/rust-lang/miri/issues/2132

rust-lang/miri,rust-lang/miri,rust-lang/miri,rust-lang/miri