Look for ignore next warning comments only from warned nodes

Instead of testing every comment in the stylesheet and marking the
next node, the warning filter walks back from each warned node over
the prefixed copies inserted before it and checks the comment there.

lib/processor.js

@@ -1,14 +1,34 @@
 let parser = require('postcss-value-parser')
 
 let Value = require('./value')
+let vendor = require('./vendor')
 let insertAreas = require('./hacks/grid-utils').insertAreas
 
 const OLD_LINEAR = /(^|[^-])linear-gradient\(\s*(top|left|right|bottom)/i
 const OLD_RADIAL = /(^|[^-])radial-gradient\(\s*\d+(\w*|%)\s+\d+(\w*|%)\s*,/i
 const IGNORE_NEXT = /(!\s*)?autoprefixer:\s*ignore\s+next(?!\s+warning)/i
 const IGNORE_NEXT_WARNING = /(!\s*)?autoprefixer:\s*ignore\s+next\s+warning/i
+const PREFIXED_VALUE = /(^|[\s,(])-\w+-/
 const GRID_REGEX = /(!\s*)?autoprefixer\s*grid:\s*(on|off|(no-)?autoplace)/i
 
+/**
+ * Is the node right after an `ignore next warning` control comment?
+ * Prefixed copies inserted before the node are skipped on the way back.
+ */
+function hasIgnoreWarningComment(node) {
+  let prev = node.prev()
+  while (
+    prev &&
+    prev.type === 'decl' &&
+    (vendor.prefix(prev.prop) || PREFIXED_VALUE.test(prev.value))
+  ) {
+    prev = prev.prev()
+  }
+  return (
+    !!prev && prev.type === 'comment' && IGNORE_NEXT_WARNING.test(prev.text)
+  )
+}
+
 const SIZES = [
   'width',
   'height',
@@ -56,13 +76,6 @@ class Processor {
    * Add necessary prefixes
    */
   add(css, result) {
-    // Mark nodes before prefixes are inserted between them and comments
-    css.walkComments(comment => {
-      if (IGNORE_NEXT_WARNING.test(comment.text) && comment.next()) {
-        comment.next()._autoprefixerIgnoreWarning = true
-      }
-    })
-
     // At-rules
     let resolution = this.prefixes.add['@resolution']
     let keyframes = this.prefixes.add['@keyframes']
@@ -393,7 +406,7 @@ class Processor {
         i.type === 'warning' &&
         i.plugin === 'autoprefixer' &&
         i.node &&
-        i.node._autoprefixerIgnoreWarning
+        hasIgnoreWarningComment(i.node)
       )
     })
   }