Build RTLs for dryrun lib opcodes; fix uninit vars and clean up code; start mul by large const idiom recognition.

Fix bug in which addressing reg is falsely seen as binary arithmetic reg, causing overflow checks to be UNSIGNED instead of UNKNOWNSIGN (false positives).

Do not allow DEF signedness to become mixed due to propagation of USE operand signedness within an RTL.

Infer signedness from opcodes; suppress overflow and underflow checks on small values from condition codes.

Removed --no-undefined flag from link step, as this flag is generally unsupported, and seems unneeded.