optimize some common muldiv cases on P1

Author: totalspectrum

Changelog.txt

@@ -1,6 +1,7 @@
 Version 7.6.2
 - Added check for `NO_COLOR` environment variable (thanks to Ada)
 - Added a warning for P1 code with _clkfreq but no _clkmode
+- Improved performance of _muldiv64 on P1 (and hence of waitus)
 - Removed spurious FIT command in P1 org/end inlines
 
 Version 7.6.1

sys/p1_code.spin

@@ -402,7 +402,10 @@ pri _div64(n, nlo, dlo) : qlo, rlo | q, r, d
     qlo := -1
     rlo := -1
     return
-    
+  if n == 0
+    qlo := nlo +/ dlo
+    rlo := nlo +// dlo
+    return
   d := rlo := r := 0
   qlo := q := 0
   repeat 64