Make ui-fulldeps --stage=1 builds in CI

src/ci/docker/host-x86_64/x86_64-gnu-llvm-15/script.sh

@@ -4,15 +4,18 @@ set -ex
 
 # Only run the stage 1 tests on merges, not on PR CI jobs.
 if [[ -z "${PR_CI_JOB}" ]]; then
-../x.py --stage 1 test --skip src/tools/tidy && \
-           # Run the `mir-opt` tests again but this time for a 32-bit target.
-           # This enforces that tests using `// EMIT_MIR_FOR_EACH_BIT_WIDTH` have
-           # both 32-bit and 64-bit outputs updated by the PR author, before
-           # the PR is approved and tested for merging.
-           # It will also detect tests lacking `// EMIT_MIR_FOR_EACH_BIT_WIDTH`,
-           # despite having different output on 32-bit vs 64-bit targets.
-           ../x.py --stage 1 test tests/mir-opt \
-                             --host='' --target=i686-unknown-linux-gnu
+     ../x.py --stage 1 test --skip src/tools/tidy && \
+     # Run the `mir-opt` tests again but this time for a 32-bit target.
+     # This enforces that tests using `// EMIT_MIR_FOR_EACH_BIT_WIDTH` have
+     # both 32-bit and 64-bit outputs updated by the PR author, before
+     # the PR is approved and tested for merging.
+     # It will also detect tests lacking `// EMIT_MIR_FOR_EACH_BIT_WIDTH`,
+     # despite having different output on 32-bit vs 64-bit targets.
+     ../x.py --stage 1 test tests/mir-opt \
+          --host='' --target=i686-unknown-linux-gnu && \
+     # Run `ui-fulldeps` in `--stage=1`, which actually uses the stage0
+     # compiler, and is sensitive to the addition of new flags.
+     ../x.py --stage 1 test tests/ui-fulldeps
 fi
 
 # NOTE: intentionally uses all of `x.py`, `x`, and `x.ps1` to make sure they all work on Linux.