[DebugInfo] Handle DBG_VALUES with multiple variable location operands in MIR

This patch adds handling for DBG_VALUE_LIST in the MIR-passes (after
finalize-isel), excluding the debug liveness passes and DWARF emission. This
most significantly affects MachineSink, which now needs to consider all used
registers of a debug value when sinking, but for most passes this change is
simply replacing getDebugOperand(0) with an iteration over all debug operands.

Differential Revision: https://reviews.llvm.org/D92578