[LegalizeVectorTypes] When widening the operands to a concat_vectors, see if...

[LegalizeVectorTypes] When widening the operands to a concat_vectors, see if we can use the widened operand 0 if the width matches and the other operands are undef.

This saves a conversion to extracts and build_vector. We already do this when both the result and the input need to be widened to the same type.

This changed the sse-intrinsics-fast-isel test because we don't lower (insert_vector_elt (scalar_to_vector X), Y, 1) well. We turn it into (vector_shuffle (scalar_to_vector X), (scalar_to_vector Y), <0, 4, 2, 3>) losing track of the fact that the upper elts could be undef.

We should probably find a way to prevent the scalarization of the <2 x f32> load on these tests.

llvm-svn: 344404