[llvm] [llubi] Add support for byte types (PR #200672)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> > Looks like uninitialized bits are not allowed in byte SSA values. Otherwise we reintroduce undef values again as multiple `bitcast b8 %undef to i8` yields different values...
> 
> Now undef bits get frozen. I'll add an experimental flag in the future to remove undef bits from memory.

It reminds me that `memcpy` copies the raw bytes in llubi (i.e., uninitialized bits are preserved). Should we freeze uninitialized bits as well for memcpy? Otherwise it is illegal to convert a copy loop into the libcall (e.g., `LoopIdiomRecognize::processLoopStoreOfLoopLoad`).

https://github.com/llvm/llvm-project/pull/200672