Teach combineMetadata how to merge 'nonnull' metadata.

combineMetadata is used when merging two instructions into one.  This change teaches it how to merge 'nonnull' - i.e. only preserve it on the new instruction if it's set on both sources.  This isn't actually used yet since I haven't adjusted any of the call sites to pass in nonnull as a 'known metadata'.  

llvm-svn: 220325