Auto merge of rust-lang#147541 - saethlin:transmute-carefully, r=scottmcm

Change int-to-ptr transmute lowering back to inttoptr

This is a revert of rust-lang#121282, but with a regression test to cover the reported miscompile in rust-lang#147265 that was caused by the way the code here combines with rust-lang#138759.

Author: bors