internal/profile: optimize Parse allocs

[austinderek]

In our case, it greatly improves the performance of continuously collecting diff profiles from the net/http/pprof endpoint, such as /debug/pprof/allocs?seconds=30.

This CL is a cherry-pick of my PR upstream: google/pprof#951

Benchmark of profile Parse func:

goos: linux
goarch: amd64
pkg: github.com/google/pprof/profile
cpu: 13th Gen Intel(R) Core(TM) i7-1360P
│ old-parse.txt │ new-parse.txt │
│ sec/op │ sec/op vs base │
Parse-16 62.07m ± 13% 55.54m ± 13% -10.52% (p=0.035 n=10)

     │ old-parse.txt │            new-parse.txt             │
     │     B/op      │     B/op      vs base                │

Parse-16 47.56Mi ± 0% 41.09Mi ± 0% -13.59% (p=0.000 n=10)

     │ old-parse.txt │            new-parse.txt            │
     │   allocs/op   │  allocs/op   vs base                │

Parse-16 272.9k ± 0% 175.8k ± 0% -35.58% (p=0.000 n=10)

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🔄 This is a mirror of upstream PR golang#76145