[nfc[[mlir][scf]: Define scf.for lower/upper bounds can be also negative or zero

Per the discussion here: #116748 (comment) , this commit properly declare that lower and upper bounds can be also negative or zero.

Author: Aviad Cohen

mlir/include/mlir/Dialect/SCF/IR/SCFOps.td

@@ -153,9 +153,10 @@ def ForOp : SCF_Op<"for",
     operation defines an SSA value for its induction variable. It has one
     region capturing the loop body. The induction variable is represented as an
     argument of this region. This SSA value is a signless integer or index.
-    The step is a value of same type but required to be positive. The lower and
-    upper bounds specify a half-open range: the range includes the lower bound
-    but does not include the upper bound.
+    The step is a value of same type but required to be positive, the lower and
+    upper bounds can be also negative or zero. The lower and upper bounds specify
+    a half-open range: the range includes the lower bound but does not include
+    the upper bound.
 
     The body region must contain exactly one block that terminates with
     `scf.yield`. Calling ForOp::build will create such a region and insert