Address a couple more UBSan warnings

[silentbicycle]

• Some of the hash functions weren't using SUPPRESS_EXPECTED_UNSIGNED_INTEGER_OVERFLOW().
• Address more implicit conversions between signed / unsigned char.
• Fix a possible underflow in ir.c.
• Explicitly cast a negated flag.
• Cast int to char in lx's generated code.

These don't address ALL the remaining warnings, but it eliminates quite a few of them.

[silentbicycle]

This CI failure should be addressed by #352.