[RegisterCoalescer] Do not call getInstructionIndex with DBG_VALUE

An assert occurs when calling SlotIndexes::getInstructionIndex with
a DBG_VALUE instruction because the function expects an instruction
with a slot index. However, there is no slot index for a DBG_VALUE
instruction.

Differential Revision: https://reviews.llvm.org/D29048

llvm-svn: 294070