[BDCE] Skip metadata while replacing uses.

The fix committed in r288851 doesn't cover all the cases.
In particular, if we have an instruction with side effects
which has a no non-dbg use not depending on the bits, we still
perform RAUW destroying the dbg.value's first argument.
Prevent metadata from being replaced here to avoid the issue.

Differential Revision:  https://reviews.llvm.org/D27534

llvm-svn: 288987