[IR] add shuffle queries for identity extend/extract

This was one of the potential follow-ups suggested in D48236, 
and these will be used to make matching the patterns in PR38691 cleaner:
https://bugs.llvm.org/show_bug.cgi?id=38691

About the vocabulary: in the DAG, these would be concat_vector with an 
undef operand or extract_subvector. Alternate names are discussed in the
review, but I think these are familiar/good enough to proceed. Once we
have uses of them in code, we might adjust if there are better options.

https://reviews.llvm.org/D51392

llvm-svn: 341075