[DebugInfo][InstrRef][4/4] Support DBG_INSTR_REF through all backend passes

This is a cleanup patch -- we're now able to support all flavours of
variable location in instruction referencing mode. This patch updates
various tests for debug instructions to be broader: numerous code paths
try to ignore debug isntructions, and they now have to ignore the
additional DBG_PHI and DBG_INSTR_REFs that we can generate.

A small amount of rework happens for LiveDebugVariables: as we don't need
to track live intervals through regalloc any more, we can get away with
unlinking debug instructions before regalloc, then re-inserting them after.
Note that this isn't (yet) true of DBG_VALUE_LISTs, they still have to go
through live interval tracking.

In SelectionDAG, add a helper lambda that emits half-formed DBG_INSTR_REFs
for arguments in instr-ref mode, DBG_VALUE otherwise. This is one of the
final locations where DBG_VALUEs are emitted for vreg arguments.

X86InstrInfo now un-sets the debug instr number on SUB instructions that
get mutated into CMP instructions. As the instruction no longer computes a
subtraction, we can't use it for variable locations.

Differential Revision: https://reviews.llvm.org/D88898