Widen ascii to utf16

[pgovind]

Ready for review now.

Implements WidenAsciiToUtf16 from #35034

Perf:

summary:
better: 1, geomean: 2.172
total diff: 1

No Slower results for the provided threshold = 0.01% and noise filter = 0.3ns.

| Faster                                                       | base/diff | Base Median (ns) | Diff Median (ns) | Modality|
| ------------------------------------------------------------ | ---------:| ----------------:| ----------------:| --------:|
| System.Text.Experimental.Perf.IsNormalized_WidenAsciiToUtf16 |      2.17 |         65021.15 |         29935.14 |         |

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[ghost]

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[kunalspathak]

Same here. Please add the benchmark in dotnet/performance.

[echesakov]

LGTM (with nits)

[pgovind]

Thanks for the reviews @eiriktsarpalis @echesakovMSFT and @kunalspathak

[pgovind]

CI failures are unrelated. Merging

[kunalspathak]

Could you double check the benchmark IsNormalized_WidenAsciiToUtf16() that you added in dotnet/performance#1445 to see if it is actually hitting the code path. You don't do anything with the return value so does JIT optimize away calls that would have otherwise called the code path you wanted to measure?

[pgovind]

Yup, it's hitting the code path. I added an exception that triggered by the benchmarking code:

// BeforeAnythingElse


Unhandled exception.
   Cannot print exception string because Exception.ToString() failed.
ExitCode != 0
// Benchmark Process 4827 has exited with code 134
No more Benchmark runs will be launched as NO measurements were obtained from the previous run!