Correct coverage for await using (issue #914)

[alexthornton1]

This commit corrects coverage for the await using statement added
in C# 8, including the variant without an explicit scope (i.e.,
without curly braces and a body), by eliminating two kinds of
patterns that arise in the async state machine that weren't
already being detected and eliminated.

[alexthornton1]

There are a couple of interesting things going on in this PR that I thought I should mention:

1. I found myself wanting to ask a question like "Does this instruction load from argument 0?" or "Does this instruction store to the same local variable that the previous instruction loaded from?", but ran into some trouble with Cecil -- which I'm not very familiar with yet -- sometimes deciding the instruction would be something like ldc.i4.1 and sometimes deciding it would be ldc.i4 1 instead, inconsistent with what I'd see in disassembly. So I added a new class in coverlet.core called InstructionExtensions that smooths out those differences. If that's working around my misunderstanding of Cecil, I'd be glad to be set straight on that.
2. I found that some of the same patterns I was looking for in async foreach occurred again in async using, so there's a refactoring opportunity available there, but I wonder if the refactoring opportunity is actually a larger one, because I see similar patterns being searched for elsewhere. For example, I needed to look for a pattern in SkipGeneratedBranchesForAwaitUsing.CheckForCleanup that seemed similar to SkipGeneratedBranchesForExceptionHandlers. But the existing code wouldn't match those patterns -- and that might partly be caused by the issue I found in my own code that seemed to require InstructionExtensions, where the code was searching for similar (but not the same) instructions and, thus, not matching.

[alexthornton1]

Before:

After:

[alexthornton1]

Okay, that's my last tweak until it's reviewed. :)

[MarcoRossignoli]

Let me know if we can avoid new helpers.
A part from that LGTM

[alexthornton1]

After commit 56d124d:

[MarcoRossignoli]

LGTM