Mac instruction error

Created by: ChunyuLiao

1. decoding conflict Decoding Conflict: 00...............101.....1011011 .................101.....1011011 .........................1011011 ................................ CV_MULUN 00_______________101_____1011011 CV_MULURN 00_______________101_____1011011 Decoding Conflict: 01...............101.....1011011 .................101.....1011011 .........................1011011 ................................ CV_MULHHUN 01_______________101_____1011011 CV_MULHHURN 01_______________101_____1011011

2. https://github.com/openhwgroup/corev-llvm-project/actions/runs/4341096331/jobs/7580286236 testcase MC/RISCV/corev/mac-all-extensions.s fail.