cmd/compile: mark block control instruction unsafe in "all unsafe" functions

Currently, for runtime functions and nosplit functions, it is
considered "all unsafe", meaning that the entire function body is
unsafe points. In the past, we didn't mark CALLs in such
functions unsafe, which is fixed in CL 230541. We also didn't
mark block control instructions (for mostly-empty blocks) unsafe.
This CL fixes it.

May fix #36110.

Change-Id: I3be8fdcef2b294e5367b31eb1c1b5e79966565fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/236597


Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>