CI: upload built peepholes as artifacts

For people who can't build Z3, this lets them update the peephole optimizers
when necessary.

.github/workflows/main.yml

@@ -165,12 +165,17 @@ jobs:
           --package peepmatic-runtime \
           --package peepmatic-test \
           --package peepmatic-souper
-    - name: Rebuild Peepmatic-based peephole optimizers and test them
+    - name: Rebuild Peepmatic-based peephole optimizers
       run: |
         cargo test \
           --features 'enable-peepmatic rebuild-peephole-optimizers' \
           peepmatic
       working-directory: ./cranelift/codegen
+    - name: Upload rebuilt peephole optimizers
+      uses: actions/upload-artifact@v2
+      with:
+        name: peephole-optimizers
+        path: cranelift/codegen/src/preopt.serialized
     - name: Check that built peephole optimizers are up to date
       run: git diff --exit-code
     - name: Test with Peepmatic-based peephole optimizers