[DebugInfo][InstrRef] Accept landingpad block arguments

This patch makes instruction-referencing accepts an additional scenario
where values can be read from physical registers at the start of blocks. As
far as I was aware, this only happened:
 * With arguments in the entry block,
 * With constant physical registers,

To which this patch adds a third case:
 * With exception-handling landing-pad blocks

In the attached test: the operand of the dbg.value traces back to the
"landingpad" instruction, which becomes some copies from physregs. Right
now, that's deemed unacceptable, and the assertion fires. The fix is to
just accept this scenario; this is a case where the value in question is
defined by a register and a position, not by an instruction that defines
it. Reading it with a DBG_PHI is the correct behaviour, there isn't a
non-copy instruction that we can refer to.

Differential Revision: https://reviews.llvm.org/D109005