Auto merge of #121282 - saethlin:gep-null-means-no-provenance, r=scottmcm

Lower transmutes from int to pointer type as gep on null

I thought of this while looking at rust-lang/rust#121242. See that PR's description for why this lowering is preferable.

The UI test that's being changed here crashes without changing the transmutes into casts. Based on that, this PR should not be merged without a crater build-and-test run.

Author: bors