[ios] Disable experimental isel.

Set -fno-experimental-isel until until https://llvm.org/PR40887 is fixed.
Otherwise, debugging on a device has buggy stepping logic.

Bug: 910200
Change-Id: I8e0df10de27403342d85d61672cc356644d49694
Reviewed-on: https://chromium-review.googlesource.com/c/1492044
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#636303}

build/config/ios/BUILD.gn

@@ -65,6 +65,12 @@ config("compiler") {
   # an Objective C struct won't be called, which is very bad.
   cflags_objcc = [ "-fobjc-call-cxx-cdtors" ]
 
+  # TODO(crbug.com/910200): Disable experimental isel until
+  # https://llvm.org/PR40887 is fixed.
+  if (is_debug) {
+    cflags += [ "-fno-experimental-isel" ]
+  }
+
   ldflags = common_ios_flags
 }