fix: avoid undesirable integral promotions with bitwise operations

The test case bitwise-not.m triggers the following warning with GCC 9:

  C compilation failed:
  <stdin>: In function ‘rule0’:
  <stdin>:3836:126: error: promoted ~unsigned is always non-zero [-Werror=sign-compare]
  <stdin>:3836:244: error: comparison of promoted ~unsigned with constant [-Werror=sign-compare]