[X86][AVX] Remove X86ISD::SUBV_BROADCAST (PR38969)

Followup to D92645 - remove the remaining places where we create X86ISD::SUBV_BROADCAST, and fold splatted vector loads to X86ISD::SUBV_BROADCAST_LOAD instead.

Remove all the X86SubVBroadcast isel patterns, including all the fallbacks for if memory folding failed.