cmd/compile/internal/ir: add "never returns" func flag
Add a flag to ir.Func's flags field to record whether a given function
is deemed to never return (e.g. always calls exit or panic or
equivalent on all control paths). So as to not increase the amount of
flag storage, this new flag replaces the existing "ExportInline" flag,
which is currently unused.
Change-Id: Idd336e47381048cfc995eda05faf8b62f06ba206
Reviewed-on: https://go-review.googlesource.com/c/go/+/518256 Reviewed-by: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>