Remove UB from indeterminate value handling

Move SIMD operations on potentially-indeterminate Node16::index bytes
into file-level assembly, where loading and operating on indeterminate
values is well-defined (unlike C++). Restructure scalar fallback loops
to iterate [0, numChildren) instead of [0, kMaxNodes). Fix TrivialSpan
construction from indeterminate pointers in check::Job::init and
insertPointWritesOrSorted to only construct when end.len > 0.

Add MSan toolchain to the debug CI build to catch these issues going
forward.

.gitea/workflows/ci.yml

@@ -40,7 +40,7 @@ jobs:
           - name: 64-bit-versions
             cmake_args: -DCMAKE_CXX_FLAGS=-DUSE_64_BIT=1
           - name: debug
-            cmake_args: -DCMAKE_BUILD_TYPE=Debug
+            cmake_args: -DCMAKE_BUILD_TYPE=Debug -DMSAN_TOOLCHAIN_PATH=/opt/msan
           - name: simd-fallback
             cmake_args: -DUSE_SIMD_FALLBACK=ON
           - name: gcc
@@ -76,6 +76,13 @@ jobs:
             sudo update-alternatives --install /usr/bin/${tool} ${tool} /usr/bin/${tool}-21 100
           done
 
+      - name: Download MSan toolchain
+        if: matrix.name == 'debug'
+        run: |
+          curl -Ls "https://minio.weaselab.dev/public/x86_64/msan-toolchain-21.1.8.tar.zst" -o /tmp/msan-toolchain.tar.zst
+          sudo mkdir -p /opt/msan
+          sudo tar --zstd -xf /tmp/msan-toolchain.tar.zst -C /opt/msan
+
       - name: Build
         run: |
           export CCACHE_DIR="$GITHUB_WORKSPACE/.ccache"