compile simple loop conditions twice for a more efficient branch stru…

…cture

I didn't think this would matter, but it actually makes a big difference
to LLVM. fixes #5469

with this, I believe no changes to Range1 are needed for performance.

src/julia-syntax.scm

@@ -2914,13 +2914,28 @@ So far only the second case can actually occur.
 			(set-car! (cdr end-jump) (make&mark-label)))))
 	    ((block) (for-each (lambda (x) (compile x break-labels vi))
 			       (cdr e)))
-	    ((_while) (let ((topl (make&mark-label))
-			    (endl (make-label)))
-			(compile (cadr e) break-labels vi)
-			(emit `(gotoifnot ,(goto-form (caddr e)) ,endl))
-			(compile (cadddr e) break-labels vi)
-			(emit `(goto ,topl))
-			(mark-label endl)))
+	    ((_while)
+	     (let ((test-blk (cadr e))
+		   (endl (make-label)))
+	       (if (or (atom? test-blk) (equal? test-blk '(block)))
+		   ;; if condition is simple, compile it twice in order
+		   ;; to generate a single branch per iteration.
+		   (let ((topl (make-label)))
+		     (compile test-blk break-labels vi)
+		     (emit `(gotoifnot ,(goto-form (caddr e)) ,endl))
+		     (mark-label topl)
+		     (compile (cadddr e) break-labels vi)
+		     (compile test-blk break-labels vi)
+		     (emit `(gotoifnot (call (top !) ,(goto-form (caddr e))) ,topl))
+		     (mark-label endl))
+
+		   (let ((topl (make&mark-label)))
+		     (compile test-blk break-labels vi)
+		     (emit `(gotoifnot ,(goto-form (caddr e)) ,endl))
+		     (compile (cadddr e) break-labels vi)
+		     (emit `(goto ,topl))
+		     (mark-label endl)))))
+
 	    ((break-block) (let ((endl (make-label)))
 			     (compile (caddr e)
 				      (cons (list (cadr e) endl handler-level)