[X86] Handle repeated blend mask in combineConcatVectorOps (#82155)

https://github.com/llvm/llvm-project/commit/1d27669e8ad07f8f2 add
support for fold 512-bit concat(blendi(x,y,c0),blendi(z,w,c1)) to
AVX512BW mask select.
But when the type of subvector is v16i16, we need to generate repeated
mask to make the result correct.
The subnode looks like t87: v16i16 = X86ISD::BLENDI t132, t58,
TargetConstant:i8<-86>.