[X86] Move a transform out of combineConcatVectorOps so we don't prematurely turn CONCAT_VECTORS into INSERT_SUBVECTORS.
[X86] Move a transform out of combineConcatVectorOps so we don't prematurely turn CONCAT_VECTORS into INSERT_SUBVECTORS.

CONCAT_VECTORS and INSERT_SUBVECTORS can both call combineConcatVectorOps,
but we shouldn't produce INSERT_SUBVECTORS from there. We should
keep CONCAT_VECTORS until vector legalization.

Noticed while looking at the madd_quad_reduction test from madd.ll

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@369802 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm