[DebugInfo][InstrRef] Correctly update DBG_PHIs during instr scheduling

Avoid several crashes when DBG_INSTR_REF and DBG_PHI instructions are fed
to the instruction scheduler. DBG_INSTR_REFs should be treated like
DBG_LABELs, and just ignored for the purpose of scheduling [0].

DBG_PHIs however behave much more like DBG_VALUEs: they refer to register
operands, and if some register defs get shuffled around during instruction
scheduling, there's a risk that the debug instr will refer to the wrong
value. There's already a facility for updating DBG_VALUEs to reflect this;
add DBG_PHI to the list of instructions that it will update.

[0] Suboptimal, but it's what instr scheduling does right now.

Differential Revision: https://reviews.llvm.org/D106663