[DebugInfo][RemoveDIs] Assert if we mix PHIs and debug-info (#84054)

A potentially erroneous code construction with the work we've done to
remove debug intrinsics, is inserting PHIs into blocks when the position
hasn't been "sourced correctly". Specifically, if you have:

    %foo = PHI
    #dbg_value
    %bar = add i32...

And plan on inserting a new PHI, you have to use the iterator form of
`getFirstNonPHI` or getFirstInsertionPt (or begin()) to acquire an
iterator that tells the debug-info maintenance code "this is supposed to
be at the start of the block, put it in front of #dbg_value". We can
detect call-sites that aren't doing this at runtime, and should do with
this assertion. It might invalidate code that's doing something very
unexpected, like walking backwards to find a PHI, then going forwards,
then inserting: however that's just an inefficient way of calling
`getFirstNonPHI`.