Accept minor code size regressions after LLVM update

[kripken]

LLVM change: llvm/llvm-project@9423f78

That uses unsigned operations in more places. Those are less size-efficient
than signed ones in wasm2js, so unfortunately this increases code size there.
I do not see a simple way to optimize things better in wasm2js - the LLVM
change just switches to an unsigned operation, but then that gets taken
into account in later work - apparently sometimes helpful, hence the change -
and the result is not a simple unsigned operation we can replace easily.