[X86] Handle multiple use freeze(undef) in LowerAVXCONCAT_VECTORS as zero vectors

Replacement for D144903

If we're concatenating freeze(undef) subvector ops with multiple uses then we can't treat them as a wider freeze(undef), but we can replace them with a zero subvector, which is cheap on AVX

Differential Revision: https://reviews.llvm.org/D149249