Fix optImpliedByTypeOfAssertions to correctly find a non-null assertion

[EgorBo]

Fixes #120792

This looks like some very old bug that was exposed by an unrelated change in .NET 10.0

Basically, we have an assertion obj->pMT == <cns_type> we invoke optImpliedByTypeOfAssertions to see if we can find some existing assertion (anywhere) for obj != null so we can duplicate it into current ASSERT_TP& activeAssertions, because obj->pMT assertion implies obj is not null.

We do find such an assertion but for some reason we don't check that it's null on the right side (op2) -> obj != null. Instead, we pick up some random obj != <frozen_object> assertion which then is incorrectly used.

No diffs.

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[Copilot]

Pull Request Overview

This PR fixes a bug in the JIT compiler's assertion propagation logic where optImpliedByTypeOfAssertions was incorrectly identifying non-null assertions. When searching for an implied obj != null assertion after finding obj->pMT == <cns_type>, the code was incorrectly accepting any obj != <constant> assertion instead of specifically verifying that the right-hand side was null.

Key Changes:

• Fixed the assertion validation logic to use CanPropNonNull() to verify the assertion is actually obj != null
• Simplified duplicate-checking logic using TryAddElemD
• Added regression test to prevent future occurrences

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/coreclr/jit/assertionprop.cpp	Fixed bug by replacing manual operand checks with CanPropNonNull() call and simplified duplicate detection
src/tests/JIT/Regression/JitBlue/Runtime_120792/Runtime_120792.cs	Added regression test that triggers the bug scenario with type assertions and null checks
src/tests/JIT/Regression/JitBlue/Runtime_120792/Runtime_120792.csproj	Test project configuration with tiered compilation and PGO enabled

[EgorBo]

cc @dotnet/jit-contrib

[EgorBo]

/backport to release/10.0-staging

[github-actions]

Started backporting to release/8.0-staging: https://github.com/dotnet/runtime/actions/runs/18679209211

[github-actions]

Started backporting to release/10.0-staging: https://github.com/dotnet/runtime/actions/runs/18679216580

[github-actions]

@EgorBo backporting to "release/8.0-staging" failed, the patch most likely resulted in conflicts:

$ git am --3way --empty=keep --ignore-whitespace --keep-non-patch changes.patch

Applying: fix optImpliedByTypeOfAssertions to correctly find non-null assertion
Using index info to reconstruct a base tree...
M	src/coreclr/jit/assertionprop.cpp
Falling back to patching base and 3-way merge...
Auto-merging src/coreclr/jit/assertionprop.cpp
CONFLICT (content): Merge conflict in src/coreclr/jit/assertionprop.cpp
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config set advice.mergeConflict false"
Patch failed at 0001 fix optImpliedByTypeOfAssertions to correctly find non-null assertion
Error: The process '/usr/bin/git' failed with exit code 128

Please backport manually!

[github-actions]

@EgorBo an error occurred while backporting to "release/10.0-staging", please check the run log for details!

Error: The specified backport target branch "release/10.0-staging" wasn't found in the repo.