[PowerPC/QPX] Fix the load/splat peephole with overlapping reads

If, in between the splat and the load (which does an implicit splat), there is
a read of the splat register, then that register must have another earlier
definition. In that case, we can't replace the load's destination register with
the splat's destination register.

Unfortunately, I don't have a small or non-fragile test case.

llvm-svn: 268152