Investigate perf gain by using the .NET char comparison pattern

[daxian-dbw]

Description of the new feature/enhancement

This is a follow-up issue tracking the discussion in #1909 (comment).
We can investigate to see if we can get some perf gain by using the .NET char comparison pattern, such as in https://source.dot.net/#System.Private.CoreLib/UnicodeUtility.cs,141

[iSazonov]

Do you want to measure only LengthInBufferCells() method?
I could measure and share results if you want.

[daxian-dbw]

Yup, measuring only the LengthInBufferCells() would be enough. Thanks!

[iSazonov]

In my test the worst case (char is in last range) is improved up to 16%. Thoughts?

BenchmarkDotNet=v0.12.1, OS=Windows 10.0.18362.1082 (1903/May2019Update/19H1)
Intel Core i5-2410M CPU 2.30GHz (Sandy Bridge), 1 CPU, 4 logical and 2 physical cores
.NET Core SDK=5.0.100-rc.2.20479.15
  [Host]     : .NET Core 5.0.0 (CoreCLR 5.0.20.47505, CoreFX 5.0.20.47505), X64 RyuJIT
  DefaultJob : .NET Core 5.0.0 (CoreCLR 5.0.20.47505, CoreFX 5.0.20.47505), X64 RyuJIT

Method	InputString	Mean	Error	StdDev	Ratio	Gen 0	Gen 1	Gen 2	Allocated	Code Size
LengthOrig	ï¿¡	8.059 ns	0.0812 ns	0.0759 ns	1.00	-	-	-	-	251 B
LengthNew	ï¿¡	6.757 ns	0.0846 ns	0.0791 ns	0.84	-	-	-	-	206 B