Fixed a bug in the PBQP allocator's findCoalesces method.

Fixed a bug in the PBQP allocator's findCoalesces method.

Previously spill registers, whose def indexes are not defined, would sometimes be improperly marked as coalescable with conflicting registers. The new findCoalesces routine conservatively assumes that any register with at least one undefined def is not coalescable with any register it interferes with.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@95636 91177308-0d34-0410-b5e6-96231b3b80d8