5291 x86 {high,low}bit rely on undefined behavior

@@ -62,19 +62,10 @@
         .inline ipltospl,0
         movq    %rdi, %rax
         .end
 
 /
-/ find the low order bit in a word
-/
-        .inline lowbit,4
-        movq    $-1, %rax
-        bsfq    %rdi, %rax
-        incq    %rax
-        .end
-
-/
 / Networking byte order functions (too bad, Intel has the wrong byte order)
 /
 
         .inline htonll,4
         movq    %rdi, %rax