Specify const for operator() in LiveRangeOrdering

This is a preparation CL to remove -Wno-user-defined-warnings warning suppression.

Bug: chromium:683349
Change-Id: If883c24ebac66292329abaf01c5d328bfd51c5c9
Reviewed-on: https://chromium-review.googlesource.com/c/1286391Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56727}

src/compiler/register-allocator.h

@@ -1041,7 +1041,7 @@ class LinearScanAllocator final : public RegisterAllocator {
 
  private:
   struct LiveRangeOrdering {
-    bool operator()(LiveRange* a, LiveRange* b) {
+    bool operator()(const LiveRange* a, const LiveRange* b) const {
       return a->ShouldBeAllocatedBefore(b);
     }
   };