[DebugInfo][NFC] Represent DbgValues with multiple ops in IRefLDV

In preparation for allowing InstrRefBasedLDV to handle DBG_VALUE_LIST,
this patch updates the internal representation that it uses to represent
debug values to store a list of values. This is one of the more
significant changes in terms of line count, but is fairly simple and
should not affect the output of this pass.

Differential Revision: https://reviews.llvm.org/D128177