[CI] Filter runs of LLVM Test workflow (#15458)

This workflow now only triggers on changes to paths that are directly
related to LLVM.
It also runs on a nightly schedule and can be triggered manually.

Co-authored-by: Sijawusz Pur Rahnama <sija@sija.pl>

.github/workflows/llvm.yml

@@ -1,6 +1,21 @@
 name: LLVM CI
 
-on: [push, pull_request]
+on:
+  push:
+    paths:
+      - 'src/llvm/**'
+      - 'spec/std/llvm/**'
+      - 'spec/llvm-ir/**'
+      - '.github/workflows/llvm.yml'
+  pull_request:
+    paths:
+      - 'src/llvm/**'
+      - 'spec/std/llvm/**'
+      - 'spec/llvm-ir/**'
+      - '.github/workflows/llvm.yml'
+  schedule:
+    - cron: '0 3 * * *'
+  workflow_dispatch:
 
 permissions: {}