[AArch64][GlobalISel] Add combine for build_vector(unmerge, unmerge, undef, undef)#165539

HolyMolyCowMan wants to merge 1 commit intollvm:mainllvm/llvm-project:mainfrom

HolyMolyCowMan:combine-unmerge-undefHolyMolyCowMan/llvm-project:combine-unmerge-undefCopy head branch name to clipboard