Rollup merge of #50246 - nnethercote:no-dump_allocs, r=Mark-Simulacrum

Make dump_{alloc,allocs,local}() no-ops when tracing is disabled.

Because they traverse data structures and build up strings, which is
wasted effort if those strings aren't printed.

The patch also removes some now-unnecessary log_enabled! tests at call
sites.

This is a big win for the Debug and Opt runs of coercions, tuple-stress, html5ever, and encoding.
```
coercions-opt
        avg: -7.8%      min: -14.8%     max: 0.1%
coercions
        avg: -8.0%      min: -12.8%     max: 0.1%
tuple-stress
        avg: -7.2%      min: -10.8%     max: -0.7%
tuple-stress-opt
        avg: -6.9%      min: -10.7%     max: 0.6%
html5ever
        avg: -4.6%      min: -7.3%      max: -0.3%
encoding
        avg: -2.4%      min: -4.5%      max: 0.1%
html5ever-opt
        avg: -2.7%      min: -4.2%      max: -0.2%
encoding-opt
        avg: -1.4%      min: -2.4%      max: 0.0%
```

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