CodeGen: Don't iterate over operands after we've erased an MI

This fixes a use-after-free introduced 3 years ago, in r182872 ;)

The code more or less worked because the memory that CopyMI was
pointing to happened to still be valid, but lots of tests would crash
if you ran under ASAN with the recycling allocator changes from
llvm.org/PR26808

llvm-svn: 264455