Add .NET 10 Preview 2 release notes -- Libraries and Runtime

[richlander]

This is the space for libraries and runtime improvements. We'll aim to merge this PR by March 7th to align with the planned ship date of March 11th.

Everyone is welcome to add feature descriptions and participate in this process.

Parent PR/branch: #9769.

[hez2010]

Adding some detailed descriptions based on my PRs for picking up if you would like.

JIT Enhancements

Devirtualization and Inlining Improvements

• Enable inlining for calls that are late devirtualized
  Late devirtualization can introduce new inline candidates. Previously, these candidates were abandoned, but they can now be marked and inlined seamlessly. Inlining a late devirtualized call can, in turn, create additional devirtualization opportunities, which can then be inlined, further enhancing optimization potential.

• Sharpen the type of return spilling temp from inlinees
  During inlining, a temporary variable may be created to hold the return value from the callee. Now, the JIT analyzes and updates the type of this temporary variable accordingly. If all return sites in a callee yield the same exact type, this precise type information is utilized for devirtualizing subsequent calls on it.

With these two enhancements, along with recent efforts on array de-abstraction, the JIT can now devirtualize, inline, stack-allocate, and then perform struct promotion on arbitrary enumerators, so that the enumerator abstraction can be entirely eliminated, even without PGO. (Example: https://godbolt.org/z/9svq156Gj)

NativeAOT Enhancements

Type Preinitializer Improvements

• Support for more opcodes in type preinitializer
  NativeAOT includes a type preinitializer that can execute type initializers (static constructors) without side effects at compile time using an IL interpreter. The results are then embedded directly into the binary, allowing the elimination of these initializers entirely. This update expands support to cover all variants of conv.* and neg opcodes, enabling preinitialization of methods that include casting or negation operations.

[richlander]

Looks great @hez2010 -- Can you create a PR on the branch?

Can you define "late devitalization" in the text? "late" can be used in multiple ways, like "late-stage capitalism". We want to make it clear to readers.

As much as possible, can you align with the format we've used in release notes previously, to match?

Also, if that's too much, I'm happy to do this myself.

[amanasifkhalid]

As much as possible, can you align with the format we've used in release notes previously, to match?

Also, if that's too much, I'm happy to do this myself.

If it's easier, I can add @hez2010's notes to my batch of JIT notes.

[richlander]

Sounds good @amanasifkhalid. Let's do that.