JIT: Enable tail await optimization in async versions in tier 0

[jakobbotsch]

Otherwise we will allocate continuations for paths that are going to be allocation free in tier 1. This shows up in traces when running TechEmpower benchmarks as a large amount of allocation happening until everything gets tiered up.

Diff on alloc metrics on platform-json:

-| Max Allocation Rate (B/sec)             | 110,580,952                             |
+| Max Allocation Rate (B/sec)             | 8,030,616                               |

FYI @BrennanConroy

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

This PR broadens the conditions under which async tail-await handling is enabled during import by switching from OptimizationEnabled() to Tier0OptimizationEnabled(), allowing the JIT to apply the same tail-await eligibility logic in Tier 0 (when Tier 0 opts are allowed) rather than waiting until Tier 1.

Changes:

• Enable tail-await setup logic for async-version calls under Tier 0 optimization settings by using opts.Tier0OptimizationEnabled() in impSetupAsyncCall.

Comments suppressed due to low confidence (1)

src/coreclr/jit/importercalls.cpp:7301

• This change enables additional tail-await handling in Tier0 by consulting canTailCall(...) during import. Since the PR is motivated by allocation/perf impact (TechEmpower startup / pre-tiering), it would be helpful to include concrete before/after data (e.g., allocation count/size and/or throughput) so reviewers can validate the net win and any potential Tier0 import-time overhead.

        if (opts.Tier0OptimizationEnabled() && ((prefixFlags & PREFIX_IS_ASYNC_VERSION_TAIL_AWAIT) != 0) &&
            (call->gtReturnType == info.compRetType))
        {
            CORINFO_METHOD_HANDLE exactCalleeHnd =
                ((call->AsCall()->gtCallType != CT_USER_FUNC) || call->AsCall()->IsVirtual()) ? nullptr : methHnd;

            asyncInfo.IsTailAwait =
                info.compCompHnd->canTailCall(info.compMethodHnd, methHnd, exactCalleeHnd, /* fIsTailPrefix */ false);
        }

[jakobbotsch]

cc @dotnet/jit-contrib PTAL @EgorBo