JIT: Remove redundant normalizing casts when passing and returning structs

[jakobbotsch]

#113178 switched returned promoted structs to always be morphed to FIELD_LIST and started handling the introduced FIELD_LIST in the backend. However, this change introduced new unnecessary normalization when returning promoted structs that previously would not happen with the old LowerRetSingleRegStructLclVar code path.

This change adds an optimization to get rid of the introduced normalization when possible.

Fix #113382

For the benchmark:

Method	Toolchain	Mean	Error	StdDev	Median	Min	Max	Ratio	Allocated	Alloc Ratio
WriteHalf	pr	3.635 ns	0.0113 ns	0.0100 ns	3.635 ns	3.616 ns	3.657 ns	0.93	-	NA
WriteHalf	main	3.913 ns	0.0146 ns	0.0137 ns	3.911 ns	3.885 ns	3.935 ns	1.00	-	NA

[jakobbotsch]

/azp run Fuzzlyn

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Azure Pipelines successfully started running 1 pipeline(s).

[Copilot]

Pull Request Overview

This change optimizes the JIT by removing redundant normalizing casts when returning promoted structs. It addresses a performance regression introduced by #113178 which forced all returned promoted structs to use FIELD_LIST morphing, leading to unnecessary normalization that didn't exist in the previous LowerRetSingleRegStructLclVar code path.

Key changes:

• Removes genActualType() call when determining register types for return field lists
• Adds optimization logic to eliminate unnecessary casts on the last register entry when returning structs

[jakobbotsch]

cc @dotnet/jit-contrib PTAL @amanasifkhalid

Opened #118377 for the Fuzzlyn issue.

Diffs. Pretty minor, but affects the micro benchmark.