X86 CodeGenPrep: sink shufflevectors before shifts

On x86, shifting a vector by a scalar is significantly cheaper than shifting a
vector by another fully general vector. Unfortunately, because SelectionDAG
operates on just one basic block at a time, the shufflevector instruction that
reveals whether the right-hand side of a shift *is* really a scalar is often
not visible to CodeGen when it's needed.

This adds another handler to CodeGenPrepare, to sink any useful shufflevector
instructions down to the basic block where they're used, predicated on a target
hook (since on other architectures, doing so will often just introduce extra
real work).

rdar://problem/16063505

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201655 91177308-0d34-0410-b5e6-96231b3b80d8
Try addding datalayout in case that's what Hexagon doesn't like.

Just a wild stab in the dark really, but in the absence of any ability to
reproduce the problem...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201658 91177308-0d34-0410-b5e6-96231b3b80d8