Optimize native code size of Dictionary methods

[jkotas]

From dotnet/coreclr#10203 (comment)

I have glanced over the native code generated for Dictionary<int,int>.Remove. The CSE for entry[i] kicks in some cases, but not others. If you cache ref Entry entry = ref entries[i]; at the start of the loop and replace all 9 occurrences of entries[i] with it, the native code generated for this method should be approximately 20% smaller.

[omariom]

We can already use C# 7 in coreclr and corefx!?

[jkotas]

Yes, for a while already.

[mikedn]

You can assign to me

[jkotas]

@mikedn Thank you!