DAGCombiner: Extend reduceBuildVecToTrunc to handle non-zero offset

Summary:
Adding support for combining power2-strided build_vector's where the
first build_vectori's operand is extracted from a non-zero index.

Example:

 v4i32 build_vector((extract_elt V, 1),
                    (extract_elt V, 3),
                    (extract_elt V, 5),
                    (extract_elt V, 7))
 -->
 v4i32 truncate (bitcast (shuffle<1,u,3,u,5,u,7,u> V, u) to v4i64)

Reviewers: delena, RKSimon, guyblank

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D35700

llvm-svn: 309108