JIT: Avoid running return value copy logic for unused return value

[jakobbotsch]

Fix #115669

Also opened #115686 as a related optimization in the area.

[Copilot]

Pull Request Overview

This PR optimizes the JIT by ensuring that return value copy logic is only executed when the return value is actually used.

• Added an additional null check for callDefInfo.DefinitionNode before copying the return value
• This change prevents unnecessary copy operations for unused return values

[jakobbotsch]

cc @dotnet/jit-contrib PTAL @AndyAyersMS

I'm not going to add regression tests for these initial issues that Fuzzlyn is finding since I think Fuzzlyn's coverage will be enough (and there will be a number of simple things found).