Give MachineMemOperand an operator<<, factoring out code from

Give MachineMemOperand an operator<<, factoring out code from
two different places for printing MachineMemOperands.

Drop the virtual from Value::dump and instead give Value a
protected virtual hook that can be overridden by subclasses
to implement custom printing. This lets printing be more
consistent, and simplifies printing of PseudoSourceValue
values.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82599 91177308-0d34-0410-b5e6-96231b3b80d8