'block->bbNatLoopNum == BasicBlock::NOT_IN_LOOP' in 'System.SpanHelpers:LastIndexOf(byref,ushort,int):int' during 'Optimize index checks'

[kunalspathak]

With below environment variables, we hit an assert:

COMPlus_JitNoHoist=1
COMPlus_JitOptRepeat=*

Assert failure(PID 39008 [0x00009860], Thread: 40432 [0x9df0]): Assertion failed 'block->bbNatLoopNum == BasicBlock::NOT_IN_LOOP' in 'System.SpanHelpers:LastIndexOf(byref,ushort,int):int' during 'Optimize index checks' (IL size 354)

    File: D:\git\runtime\src\coreclr\jit\fgdiagnostic.cpp Line: 3775
    Image: D:\git\runtime\artifacts\tests\coreclr\windows.x64.Checked\tests\Core_Root\CoreRun.exe

[kunalspathak]

@dotnet/jit-contrib