machinst x64: lower Ctz using the Bsf x86 instruction

cranelift/codegen/src/isa/x64/inst/args.rs

@@ -293,12 +293,15 @@ impl ToString for AluRmiROpcode {
 pub enum ReadOnlyGprRmROpcode {
     /// Bit-scan reverse.
     Bsr,
+    /// Bit-scan forward.
+    Bsf,
 }
 
 impl fmt::Debug for ReadOnlyGprRmROpcode {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         match self {
             ReadOnlyGprRmROpcode::Bsr => write!(fmt, "bsr"),
+            ReadOnlyGprRmROpcode::Bsf => write!(fmt, "bsf"),
         }
     }
 }

cranelift/codegen/src/isa/x64/inst/emit.rs

@@ -566,6 +566,7 @@ pub(crate) fn emit(
 
             let (opcode, num_opcodes) = match op {
                 ReadOnlyGprRmROpcode::Bsr => (0x0fbd, 2),
+                ReadOnlyGprRmROpcode::Bsf => (0x0fbc, 2),
             };
 
             match src {