Fix ARM64 profile disassembly CFG parsing

[Copilot]

ARM64 binaries loaded from profiles could be parsed using the session’s default x64 ASM IR mode. That prevented ARM64 branches from being recognized, so basic blocks and CFG edges were not built correctly.

• Architecture-aware disassembly parsing
  • Selects the ASM parser mode from the loaded PE binary architecture for binary-backed disassembly sections.
  • Uses IRMode.ARM64 for ARM64 binaries even when the profile session initially defaults to x64.

return binaryInfo?.Architecture switch {
  Machine.Arm64 => new ASMCompilerIRInfo(IRMode.ARM64),
  Machine.I386  => new ASMCompilerIRInfo(IRMode.x86_64),
  Machine.Amd64 => new ASMCompilerIRInfo(IRMode.x86_64),
  _             => compilerInfo.IR
};

• Regression coverage
  • Adds an ARM64 parser test for a conditional branch.
  • Verifies the parser creates separate basic blocks and connects both branch target and fall-through CFG edges.

[jhpohovey]

Made a small test arm64 binary to test this; first image is off of the current HEAD of main, we see that the Flow graph is just one node.

Rebuilding with the changes in this PR, we now see that there are the individual blocks all showing up in the flow graph view:

This appears to solve the linked issue.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Fixes ARM64 disassembly parsing by selecting the ASM IR mode based on the loaded PE binary's architecture, so ARM64 branches are correctly recognized when building basic blocks and CFG edges.

Changes:

• Add SelectIRInfo helper to pick IRMode.ARM64 or IRMode.x86_64 based on PE architecture.
• Initialize DisassemblerSectionLoader with the architecture-appropriate ICompilerIRInfo.
• Add ARM64 conditional branch parser test verifying basic block split and CFG edges.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
src/ProfileExplorerCore/Compilers/ASM/DisassemblerSectionLoader.cs	Selects IR info based on PE architecture before Initialize.
src/ProfileExplorerCoreTests/ASMParserTests.cs	New test covering ARM64 conditional branch CFG creation.

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