Reduce RuntimeType.MakeGenericType overheads

[stephentoub]

• Avoid an extra GetGenericArguments() call for all arities.
• Special-case a Type[] with just one type. In looking at all calls to MakeGenericType when starting up a basic ASP.NET MVC app, 70% of the few hundred were for a single generic argument (the rest were for two).

Method	Toolchain	Mean	Ratio	Allocated
OneRefType	\master\corerun.exe	255.3 ns	1.00	128 B
OneRefType	\pr\corerun.exe	156.6 ns	0.61	32 B
OneValueType	\master\corerun.exe	247.8 ns	1.00	128 B
OneValueType	\pr\corerun.exe	161.0 ns	0.65	32 B
TwoRefType	\master\corerun.exe	285.2 ns	1.00	160 B
TwoRefType	\pr\corerun.exe	199.8 ns	0.70	120 B
TwoValueType	\master\corerun.exe	292.4 ns	1.00	160 B
TwoValueType	\pr\corerun.exe	210.4 ns	0.72	120 B

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Diagnosers;
using BenchmarkDotNet.Running;
using System;
using System.Collections.Generic;

[MemoryDiagnoser]
public class Program
{
    static void Main(string[] args) => BenchmarkSwitcher.FromAssemblies(new[] { typeof(Program).Assembly }).Run(args);

    private Type[] _oneRef = new[] { typeof(string) };
    private Type[] _oneValue = new[] { typeof(int) };
    private Type[] _twoRef = new[] { typeof(string), typeof(string) };
    private Type[] _twoValue = new[] { typeof(int), typeof(int) };

    [Benchmark] public Type OneRefType() => typeof(List<>).MakeGenericType(_oneRef);
    [Benchmark] public Type OneValueType() => typeof(List<>).MakeGenericType(_oneValue);
    [Benchmark] public Type TwoRefType() => typeof(Dictionary<,>).MakeGenericType(_twoRef);
    [Benchmark] public Type TwoValueType() => typeof(Dictionary<,>).MakeGenericType(_twoValue);
}

Contributes to #44598

[Dotnet-GitSync-Bot]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[jkotas]

LGTM

[GrabYourPitchforks]

/cc @steveharter, since it follows a similar pattern that we reviewed for #45085.