[InstCombine] combine a shuffle and an extract subvector shuffle

This is part of the missing IR-level folding noted in D52912.
This should be ok as a canonicalization because the new shuffle mask can't
be any more complicated than the existing shuffle mask. If there's some 
target where the shorter vector shuffle is not legal, it should just end up 
expanding to something like the pair of shuffles that we're starting with here.

Differential Revision: https://reviews.llvm.org/D53037

llvm-svn: 344476