Turn off Vectorization for Emscripten

When targeting Emscripten, rustc emits Vector Instructions by default.
However Web Assembly doesn't support Vector Instructions yet, which
causes Binaryen to fail converting the intermediate asm.js code to Web
Assembly. While asm.js kind of supports Vector Instructions, they
aren't supported by any browser other than Firefox, often meaning that
they need to be emulated very slowly. So it should just be turned off
for all Emscripten targets.

Fixes #38558

src/librustc_trans/back/write.rs

@@ -315,11 +315,15 @@ impl ModuleConfig {
         // Copy what clang does by turning on loop vectorization at O2 and
         // slp vectorization at O3. Otherwise configure other optimization aspects
         // of this pass manager builder.
+        // Turn off vectorization for emscripten, as it's not very well supported.
         self.vectorize_loop = !sess.opts.cg.no_vectorize_loops &&
                              (sess.opts.optimize == config::OptLevel::Default ||
-                              sess.opts.optimize == config::OptLevel::Aggressive);
+                              sess.opts.optimize == config::OptLevel::Aggressive) &&
+                             !sess.target.target.options.is_like_emscripten;
+
         self.vectorize_slp = !sess.opts.cg.no_vectorize_slp &&
-                            sess.opts.optimize == config::OptLevel::Aggressive;
+                            sess.opts.optimize == config::OptLevel::Aggressive &&
+                            !sess.target.target.options.is_like_emscripten;
 
         self.merge_functions = sess.opts.optimize == config::OptLevel::Default ||
                                sess.opts.optimize == config::OptLevel::Aggressive;