Fix UBSan error with overflowing exponent operator (#1727)

test/asm/exponent.asm

@@ -0,0 +1,11 @@
+def n = 30
+for x, -n, n
+  for p, n
+    def v1 = x ** p
+    def v2 = 1
+    for i, p
+      def v2 *= x
+    endr
+    assert v1 == v2, "{d:x}**{d:p} = {d:v1} or {d:v2}?"
+  endr
+endr