Auto merge of #130998 - the8472:bail-before-memcpy, r=nnethercote

avoid phi node for pointers flowing into Vec appends

Elide temporary allocations in patterns like `vec.append(slice.to_vec())`

related discussion: https://rust-lang.zulipchat.com/#narrow/stream/187780-t-compiler.2Fwg-llvm/topic/nocapture.20and.20allocation.20elimination

Author: bors