Improve x86 flag lifting semantics

Expand x86 LLIL flag modeling for arithmetic, test, rotate, shift,
double-shift, and signed multiply instructions. Add popcnt-based parity
calculation, auxiliary-carry formulas, SBB/ADC carry and overflow handling,
IMUL truncation checks, and count-sensitive CF/OF behavior for shifts and
rotates. Use LLIL unknown for architecturally undefined flags.

Fix several flag correctness issues, including PTEST/VPTEST flag polarity and
cleared flags, CCMP/CTEST signed-not-less conditions, EVEX default flag values,
and SHLD/SHRD count/result flag behavior.

Keep variable-count register shift and rotate value lifts clean by moving
count-zero/count-one flag handling into flag callbacks, while preserving
count-zero no-op behavior for memory destinations. Also improve ENTER lifting
for nonzero nesting levels.

Author: zznop