Add support for MCS 2.6.4 pinned region with array variable

[ElektroKill]

Link to issue(s) this covers:
N/A

Problem

The decompiler did not support decompiling pinned regions with array variables when the compiler used was MCS 2.6.4.

Before:

public unsafe int MultipleExitsOutOfFixedBlock(int[] arr)
{
	fixed (int* ptr = &System.Runtime.CompilerServices.Unsafe.AsRef<int>((int*)System.Runtime.CompilerServices.Unsafe.AsPointer(ref arr[0])))
	{
		if (*ptr < 0)
		{
			return *ptr;
		}
		if (*ptr == 21)
		{
			return 42;
		}
		if (*ptr == 42)
		{
			Console.WriteLine("outside");
			return 2;
		}
	}
	ptr = null;
	return 1;
}

After:

public unsafe int MultipleExitsOutOfFixedBlock(int[] arr)
{
	fixed (int* ptr = &arr[0])
	{
		if (*ptr < 0)
		{
			return *ptr;
		}
		if (*ptr == 21)
		{
			return 42;
		}
		if (*ptr == 42)
		{
			Console.WriteLine("outside");
			return 2;
		}
	}
	return 1;
}

Solution

• Added additional code to remove the conv instruction present in the initialization part of the pinned region.
• Extended the code responsible for removing the unpin stloc to correctly match the inverted condition found in MCS 2.6.4 compiled code.
• Enabled already present correctness test to run for MCS 2.6.4.

[dgrunwald]

If I understand correctly, the point of this change is to check both outgoing edges for an if block?
Should innerBlock.Instructions.Count == 2 be innerBlock.Instructions.Count >= 2 instead?

[ElektroKill]

Yes, I used == 2 since in my testing there were always two instructions with one being an if and the second being the fallthrough br. If we were to extend the limit to more than 2 instructions I think it would be necessary to check other instructions for potential outgoing edges which would mean additional logic. though in my testing it seems that the current code is enough.