[DebugInfo] Make postra sinking of DBG_VALUEs subregister-safe

Currently the machine instruction sinker identifies DBG_VALUE insts that
also need to sink by comparing register numbers. Unfortunately this isn't
safe, because (after register allocation) a DBG_VALUE may read a register
that aliases what's being sunk. To fix this, identify the DBG_VALUEs that
need to sink by recording & examining their register units. Register units
gives us the following guarantee:

  "Two registers overlap if and only if they have a common register unit"
  [MCRegisterInfo.h]

Thus we can always identify aliasing DBG_VALUEs if the set of register
units read by the DBG_VALUE, and the register units of the instruction
being sunk, intersect. (MachineSink already uses classes like
"LiveRegUnits" for determining sinking validity anyway).

The test added checks for super and subregister DBG_VALUE reads of a sunk
copy being sunk as well.

Differential Revision: https://reviews.llvm.org/D58191

llvm-svn: 369247