[DAG] Extend visitSCALAR_TO_VECTOR optimization to truncated vector.

[DAG] Extend visitSCALAR_TO_VECTOR optimization to truncated vector.

Relanding after case to insert explicit truncation as necessary.

Allow SCALAR_TO_VECTOR of EXTRACT_VECTOR_ELT to reduce to
EXTRACT_SUBVECTOR of vector shuffle when output is smaller. Marginally
improves vector shuffle computations.

Reviewers: efriedma, RKSimon, spatel

Subscribers: javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D35566

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310256 91177308-0d34-0410-b5e6-96231b3b80d8