[Linux/arm64] GitHub_18497 fails with assert

[sdmaclea]

corerun GitHub_18497.exe

Assert failure(PID 38848 [0x000097c0], Thread: 38848 [0x97c0]): 
Assertion failed 'varDsc->lvFieldCnt == 1' in 'GitHub_18497:Sum(struct):struct' (IL size 18)

    File: src/jit/lclvars.cpp Line: 2020

@CarolEidt To track new failure introduced in dotnet/coreclr#18499

[CarolEidt]

This is a pre-existing bug exposed by this new test.
I believe that the problem is that the struct promotion logic (where the assert is firing) is not considering the implicitByRef (passed by reference) case.