[DAGCombiner] SCALAR_TO_VECTOR(EXTRACT_VECTOR_ELT(V,C)) -> VECTOR_SHUFFLE

This patch attempts to convert a SCALAR_TO_VECTOR using an operand from an EXTRACT_VECTOR_ELT into a VECTOR_SHUFFLE.

This prevents many cases of spilling scalar data between the gpr + simd registers. 

At present the optimization only accepts cases where there is no TRUNC of the scalar type (i.e. all types must match).

Differential Revision: http://reviews.llvm.org/D8132

llvm-svn: 231554