MCP: Fixed bug with dest overlapping copy source
In MachineCopyPropagation, when propagating the source of a copy into
the operand of a later instruction, bail if a destination overlaps
(partly defines) the copy source. If the instruction where the
substitution is happening is also a copy, allowing the propagation
confuses the tracking mechanism.
Differential Revision: https://reviews.llvm.org/D69953