cmd/compile: allow inlining of open-coded defer calls

[prattmic]

Inlining happens before SSA conversion, while open-coded defers happen after, thus the open-coded deferred call can never be inlined. It would be nice to fix this.

@danscales:

It definitely is not trivial, since inlining happens before SSA conversion, whereas there are a bunch of reasons why at least some of the open-coded defer work have to be done during SSA conversion. Among other things, we need to force storing defer args to stack slots and keeping those stacks slots live even though they sometimes appear to be dead (but are actually needed to run the open-coded defers when there is a panic).

@randall77 @aclements

[prattmic]

For reference, the difference I see for mutex unlock looks like:

name          time/op
Bare-12       10.2ns ± 1%
OpenDefer-12  12.0ns ± 2%
HeapDefer-12  36.5ns ± 2%

https://play.golang.org/p/JDxJC_9hUgO

[gopherbot]

Change https://go.dev/cl/424920 mentions this issue: go/token: make mutex locking in unpack cheaper