better code generated for async functions which are never called with `async`

[andrewrk]

if an async function is never called with async then a few optimizations can be made:

• the return does not need to be atomic
• it can be assumed that these are always available: the awaiter ptr, return ptr if applicable, error return trace ptr if applicable.
• ~it can be assumed that it is never cancelled~ zig no longer has cancellation, but if it were reintroduced, this would be relevant.

Probably this should only be done for stage2. No reason to make stage1 more complicated.