[MachineCopyPropagation] In isNopCopy, check the destination registers match...

[MachineCopyPropagation] In isNopCopy, check the destination registers match in addition to the source registers.

Previously if the source match we asserted that the destination
matched. But GPR <-> mask register copies on X86 can violate this
since we use the same K-registers for multiple sizes.

Fixes this ISPC issue https://github.com/ispc/ispc/issues/1851

Differential Revision: https://reviews.llvm.org/D86507