[clang] [clang] Better bitfield access units (PR #65742)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

> > Note that changing the memory accesses performed by clang (load or store) _is_ an ABI change at IR level because of UB.
> 
> Could you clarify why UB is a consideration here?

There's some weirdness with poison values; see https://reviews.llvm.org/D129541 etc.  This is mostly an IR modeling thing; the machine instructions work fine, we just need to make sure we don't accidentally poison IR values that aren't supposed to be poison.

https://github.com/llvm/llvm-project/pull/65742