[DebugInfo][InstrRef] Avoid a crash during DBG_PHI maintenence

With D110105, the isDebug flag for register uses is now a proxy for whether
the instruction is a debug instruction; that causes DBG_PHIs to have their
operands updated by calls to updateDbgUsersToReg, which is the correct
behaviour. However: that function only expects to receive DBG_VALUE
instructions and asserts such.

This patch splits the updating-action into a lambda, and applies it to the
appropriate operands for each kind of debug instruction. Tested with an
ARM test that stimulates this function: I've added some DBG_PHI
instructions that should be updated in the same way as DBG_VALUEs.

Differential Revision: https://reviews.llvm.org/D108641