Reduce code size of Stack.Peek

[jamesqo]

Same as #12062, but for Stack. I had to add AggressiveInlining to get it to work, since it seems like the 1 extra byte from the dec in _size - 1 was making it ineligible for inlining.

Test code, before/after asm is available at the link. You can see inlining Stack.Peek increases the code size of Main by 31 bytes, while inlining Queue.Peek (asm is in the previous PR) increases it by 30.

cc @stephentoub, @ianhays

[stephentoub]

Separating out the throw into its own method is fine, keeping it the same as queue. But we shouldn't add AggressiveInlining unless there's a strong performance-motivated scenario for doing so. Getting something to inline isn't a goal in and of itself.

[justinvp]

Nit: Any reason for the naming inconsistency between ThrowForEmptyQueue in #12062 and ThrowInvalidOperationEmptyStack here? It'd be nice if both methods were named consistently. I like the naming you have chosen here, so ThrowInvalidOperationEmptyQueue and ThrowInvalidOperationEmptyStack.

[jamesqo]

Actually, I'm partial to the one I had in the other PR. Although it's less descriptive, it's more terse. I'll remove the inconsistency if this will be merged.

[jamesqo]

@stephentoub OK, sounds reasonable. I'll remove the AggressiveInlining part of this PR tmrw and only include the part separating the throw logic into a new method. If I come up with a 'strong performance-motivated scenario' another day I'll resubmit the rest of the changes.