JIT: Fix handling for SIMD types in async transformation

[jakobbotsch]

Fix #115613

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[Copilot]

Pull Request Overview

This PR fixes SIMD type handling in the JIT async transformation, updates test warning suppression, and adds debug-phase tracking.

• Replace block indirections/stores with load/store value nodes for struct and SIMD types in async.cpp.
• Centralize CS1998 warning suppression in Directory.Build.props and remove per-file pragmas.
• Add a debug-only assignment to record the “most recently active” async phase.

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/tests/async/pgo/pgo.cs	Removed local #pragma warning disable CS1998.
src/tests/async/mincallcost-microbench/mincallcost-microbench.cs	Removed local #pragma warning disable CS1998.
src/tests/async/fibonacci-without-yields-config-await/fibonacci-without-yields-config-await.cs	Removed local #pragma warning disable CS1998 comment.
src/tests/async/Directory.Build.props	Added CS1998 to global NoWarn to suppress warnings.
src/coreclr/jit/async.cpp	Switched gtNewBlkIndir/gtNewStoreBlkNode to gtNewLoadValueNode/gtNewStoreValueNode, updated assert, and added INDEBUG phase tracking.

[jakobbotsch]

cc @dotnet/jit-contrib PTAL @AndyAyersMS

Also disable the "method lacks await" warning in the async tests. I was adding a test here previously but ended up removing it, but I think those changes are ok anyway.