2 small simplifications in member access

[josh11b]

• IsInstanceMethod can look in the function's self_param_id instead of iterating through all of the implicit parameter patterns.
• An associated entity is always associated with a single interface, so we don't need to handle the case when it isn't.

[jonmeow]

Can you please update the PR title to give a brief mention of what's being simplified?

[jonmeow]

LG, but would appreciate a clearer PR title, and maybe a brief description, before merging.

[josh11b]

LG, but would appreciate a clearer PR title, and maybe a brief description, before merging.

Done.

[jonmeow]

LG, but would appreciate a clearer PR title, and maybe a brief description, before merging.

Done.

PR title doesn't appear to have changed?

[jonmeow]

Note, I'm not expecting much -- like "2 small simplifications in member access" would be fine. Just trying to have it noted in the PR description which area of the code base is affected, for anybody skimming PR titles.