LSRA assert when running PMI on xunit.performance.execution.dll

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Repro steps:

1. Build release x64 core_root and set release_core_root to its location.
2. Copy checked clrjit.dll to %release_core_root%.
3. Build pmi.dll in http://github.com/dotnet/jitutils
4. set COMPlus_TieredCompilation=0
5. %release_core_root% pmi.dll PREPALL_QUIET %release_core_root%\xunit.performance.execution.dll

This will fail. The assert won't be printed to the console because this is using a release coreclr.dll. In the debugger the assert can be seen:

<GetIterations>d__17:MoveNext():bool:this - Assertion failed (F:\runtime1\src\coreclr\src\jit\lsra.cpp:2262 - !foundDiff) during Linear scan register alloc

This failure happens when running our standard jit-diffs --pmi --frameworks with release core_root and a checked clrjit.dll. Note that it's essential that release System.Private.CoreLib.dll is used. The assert doesn't repro with checked System.Private.CoreLib.dll. Because of that, checked coreclr.dll can't be used to repro either.

[erozenfeld]

@CarolEidt @dotnet/jit-contrib