[X86][AVX] Fold loads + splats into broadcast instructions

On AVX and AVX2, BROADCAST instructions can load a scalar into all elements of a target vector.

This patch improves the lowering of 'splat' shuffles of a loaded vector into a broadcast - currently the lowering only works for cases where we are splatting the zero'th element, which is now generalised to any element.

Fix for PR23022

Differential Revision: http://reviews.llvm.org/D15310

llvm-svn: 255061