StackColoring: don't clear an instruction's mem operand if the underlying
object is a PseudoSourceValue and PseudoSourceValue::isConstant returns true (i.e.,
points to memory that has a constant value).



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181751 91177308-0d34-0410-b5e6-96231b3b80d8