Handle reference counts in one function: release().

Handle reference counts in one function: release().

This new function will decrement the reference count, and collapse a
domain value when the last reference is gone.

This simplifies DomainValue reference counting, and decouples it from
the LiveRegs array.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144131 91177308-0d34-0410-b5e6-96231b3b80d8