[X86][AVX] combineConcatVectorOps - use getBROADCAST_LOAD helper. NFCI.

This is part of rG1cfecf4f that was reverted to fix PR51226 - concating the broadcasts is OK, its the splatted loads that crash (we're not detecting extloads). I'm still creating a reduced test case so haven't added the load handling again yet.