[X86][AVX] Combine shuffles extracted from a common vector

A small step towards combining shuffles across vector sizes - this recognizes when a shuffle's operands are all extracted from the same larger source and tries to combine to an unary shuffle of that source instead. Fixes one of the test cases from PR34380.

Differential Revision: https://reviews.llvm.org/D60512

llvm-svn: 359292