Remove Peepmatic!!!

Peepmatic was an early attempt at a DSL for peephole optimizations, with the
idea that maybe sometime in the future we could user it for instruction
selection as well. It didn't really pan out, however:

* Peepmatic wasn't quite flexible enough, and adding new operators or snippets
  of code implemented externally in Rust was a bit of a pain.

* The performance was never competitive with the hand-written peephole
  optimizers. It was *very* size efficient, but that came at the cost of
  run-time efficiency. Everything was table-based and interpreted, rather than
  generating any Rust code.

Ultimately, because of these reasons, we never turned Peepmatic on by default.

These days, we just landed the ISLE domain-specific language, and it is better
suited than Peepmatic for all the things that Peepmatic was originally designed
to do. It is more flexible and easy to integrate with external Rust code. It is
has better time efficiency, meeting or even beating hand-written code. I think a
small part of the reason why ISLE excels in these things is because its design
was informed by Peepmatic's failures. I still plan on continuing Peepmatic's
mission to make Cranelift's peephole optimizer passes generated from DSL rewrite
rules, but using ISLE instead of Peepmatic.

Thank you Peepmatic, rest in peace!

.github/workflows/main.yml

@@ -88,8 +88,7 @@ jobs:
         cargo doc --no-deps --workspace \
           --exclude wasmtime-cli \
           --exclude test-programs \
-          --exclude cranelift-codegen-meta \
-          --exclude 'peepmatic*'
+          --exclude cranelift-codegen-meta
     - run: cargo doc --package cranelift-codegen-meta --document-private-items
 
     # Assemble the documentation, and always upload it as an artifact for
@@ -204,33 +203,6 @@ jobs:
     - name: Check that the ISLE DSL files are up-to-date
       run: git diff --exit-code
 
-  rebuild_peephole_optimizers:
-    name: Rebuild Peephole Optimizers
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/checkout@v2
-      with:
-        submodules: true
-    - run: rustup update stable && rustup default stable
-    - name: Test `peepmatic`
-      run: cargo test --package 'peepmatic*'
-    - 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
-      run: cargo test --features 'enable-peepmatic'
-      working-directory: ./cranelift
-
   # Perform all tests (debug mode) for `wasmtime`. This runs stable/beta/nightly
   # channels of Rust as well as macOS/Linux/Windows.
   test: