Register allocators can't handle non-uniform register sets
Neither the linear scan register allocator nor the graph-colouring allocator can properly handle the fact that some registers on x86 have 8 and 16-bit versions and some don't. We got away with this until now because the only free registers on x86 were
%edx, but now we can also treat
%esi as free when it isn't being used for R1 (see f857f074). However,
%esi doesn't have an 8-bit version, so we cannot treat it as allocatable because the register allocator will try to use it when an 8-bit register is needed (see 10575479).
LLVM doesn't have this problem, so one workaround is to compile with
-fllvm to get the extra register(s) on x86.