[DAG] reduceBuildVecExtToExtBuildVec - don't combine if it would break a splat.

reduceBuildVecExtToExtBuildVec was breaking a splat(zext(x)) pattern into buildvector(x, 0, x, 0, ..) resulting in much more complex insert+shuffle codegen.

We already go to some lengths to avoid this in SimplifyDemandedVectorElts etc. when we encounter splat buildvectors.

It should be OK to fold all splat(aext(x)) patterns - we might need to tighten this if we find a case where we mustn't introduce a buildvector(x, undef, x, undef, ..) but I can't find one.

Fixes PR46461.