Optimize shufflevector that copies an i64/f64 and zeros the rest.

Summary:
Also ran clang-format on the function. The code added is the last else
if block.

Reviewers: nadav, craig.topper

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D3518

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207992 91177308-0d34-0410-b5e6-96231b3b80d8