Use of the "h" register slices (bits 8..15) by compilers is thankfully pretty rare

That's unfortunate, because it's precisely why things like this will keep happening.

Agner Fog's optimization guide says "Any use of the high 8-bit registers AH, BH, CH, DH should be avoided because it can cause false dependences and less efficient code."

The sad vicious cycle of compilers not exercising the hardware, and then the hardware designers not paying attention. Using the high 8-bit registers and "implicitly merging" them is one of the ways to reduce the number of instructions and thus improve size optimisation.