[Bug target/118342] `a == 0 ? 32 : __builtin_ctz(a)` for Intel and AMD cores could be implemented even without BMI1

[Mayshao-oc at zhaoxin dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118342

--- Comment #3 from Mayshao-oc at zhaoxin dot com ---
Zhaoxin could confirm that for the bsf instruction, if the source register is
zero, the destination register is unchanged.