Looking at the optimization pipeline in Compiler Explorer (https://godbolt.org/z/rd3qber3b) after the code is converted to SSA (SROAPass), it seems that the fast version computes the pointer increment (`lwr++`) before storing `x` (`*lwr = x`), whereas the slow version does it the other way around. This happens because Clang chooses a different instruction order when generating a single statement compared to two statements.
The computation is the same, but apparently, this tiny change prevents SimplifyCFGPass from turning the code into the branchless version later on. I'm not sure why this happens, perhaps because it messes something up in the pattern recognition of the pass?