Auto merge of #138634 - saethlin:repeated-uninit, r=scottmcm,oli-obk

Lower to a memset(undef) when Rvalue::Repeat repeats uninit

Fixes rust-lang/rust#138625.

It is technically correct to just do nothing. But if we actually do nothing, we may miss that this is de-initializing something, so instead we just lower to a single memset that writes undef. This is still superior to the memcpy loop, in both quality of code we hand to the backend and LLVM's final output.

Author: bors