Use BBF_IMPORTER for new blocks in helperexpansion.cpp

[EgorBo]

Fixes #116143

Both "Expand runtime lookup" and "Expand static cctors" (NativeAOT) may run in Tier0/MinOpts (Debug). Convert BBF_INTERNAL to BBF_IMPORTER for new blocks (as was suggested by @eterekhin)

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Pull Request Overview

This PR updates the flag conversion logic in helperexpansion.cpp to use BBF_IMPORTED for new blocks, addressing the issue raised in #116143.

• Removed an unused variable (sigNode) in fgExpandRuntimeLookupsForCall.
• Updated BBF_INTERNAL flag conversion to BBF_IMPORTED in both runtime lookup and static initializer expansion paths.

[EgorBo]

PTAL @jakobbotsch @noahfalk

I only touched the phases that may introduce internal blocks for Tier0/MinOpts - there are a few more places for optimized code, do we need to handle them as well?

[eterekhin]

@EgorBo, Thank you for working on that!

I checked with this commit, unfortunately ICorDebugILFrame::GetIP still returns NO_MAPPING result on my code snippet and NO_MAP entry is there.

Attached a piece of ConfigureFormattingUiTestBase2<>.Test10 JIT dump

*************** In genIPmappingGen()
IP mapping count : 9
IL offs PROLOG : 0x00000000 ( STACK_EMPTY )
IL offs NO_MAP : 0x0000001C ( STACK_EMPTY )
IL offs 0x0000 : 0x0000002A ( STACK_EMPTY )
IL offs 0x0001 : 0x0000002B ( STACK_EMPTY )
IL offs NO_MAP : 0x00000055 ( STACK_EMPTY ) // <---
IL offs 0x000B : 0x00000079 ( CALL_INSTRUCTION )
IL offs 0x0010 : 0x00000082
IL offs 0x0011 : 0x00000083 ( STACK_EMPTY )
IL offs EPILOG : 0x00000084 ( STACK_EMPTY )

[noahfalk]

there are a few more places for optimized code, do we need to handle them as well?

For optimized code the long-standing convention has been the JIT would make "best effort" to produce good mapping information in optimized code, but there is no guarantee on it. My preference would be that we fix it in optimized code too assuming it isn't particularly onerous or risky to do.

[EgorBo]

@eterekhin sorry, this fell off my radar, I've just updated it.

@dotnet/jit-contrib @AndyAyersMS PTAL

It seems like for optimized codegen we have tons of places where we don't propagate the flag properly. Since we're late in the cycle, I've pushed the fix for this particular problem for Debug codegen (since we expand runtime lookups pretty much always regardless of opts). We also do static cctor initialization expansion always (on NativeAOT at least).

[eterekhin]

@EgorBo, I can confirm, the fix is working, thank you!