[llvm-reduce] Skip replacing metadata and callee operands

Metadata operands tend to require special conditions, especially on dbg
intrinsics. We also don't have a zero value for metadata.

Replacing callee operands is a little weird, since calling undef/null
doesn't make sense. It also causes tons of invalid reductions when
reducing calls to intrinsics since only arguments to intrinsics can be
of the metadata type.

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D113532