JitStress: assert(m_CallsiteDepth > 0)

[sandreenko]

I suspect this PR has caused new failures like (jitstress=1):

Assert failure(PID 2440 [0x00000988], Thread: 9332 [0x2474]): Assertion failed 'm_CallsiteDepth > 0' in '<>c:<GetType>b__1_0(System.String):System.Type:this' during 'Morph - Inlining' (IL size 7)

    File: F:\git\runtime\src\coreclr\src\jit\inlinepolicy.cpp Line: 388
    Image: F:\git\runtime\artifacts\bin\testhost\net5.0-Windows_NT-Release-x64\dotnet.exe

see https://dev.azure.com/dnceng/public/_build/results?buildId=701636&view=ms.vss-test-web.build-test-results-tab&runId=21759702&paneView=debug&resultId=105581 for more examples.

Originally posted by @sandreenko in #38163 (comment)