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

[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

llvm-svn: 369802