Rollup merge of #143718 - scottmcm:ub-transmute-is-ub, r=WaffleLapkin

Make UB transmutes really UB in LLVM

Ralf suggested in <rust-lang/rust#143410 (comment)> that UB transmutes shouldn't be trapping, which happened for the one path *that* PR was changing, but there's another path as well, so *this* PR changes that other path to match.

r? codegen

Author: matthiaskrgr