SelectionDAG: Stop using DIVariable::isInlinedFnArgument()

Instead of calling the somewhat confusingly-named
`DIVariable::isInlinedFnArgument()`, do the check directly here.
There's possibly a small functionality change here: instead of
`dyn_cast<>`'ing `DV->getScope()` to `MDSubprogram`, I'm looking up the
scope chain for the actual subprogram.  I suspect that this is a no-op
for function arguments so in practise there isn't a real difference.

I've also added a `FIXME` to check the `inlinedAt:` chain instead, since
I wonder if that would be more reliable than the
`MDSubprogram::describes()` function.

Since this was the only user of `DIVariable::isInlinedFnArgument()`,
delete it.

llvm-svn: 234799