llvm6: Don't clone LLVM modules on wasm

The comment for why cloning exists doesn't actually apply for wasm today and
apparently cloning is causing subtle bugs in LLVM, so let's just avoid it
altogether. More specifically after we emit the assembly for the wasm target we
don't actually use the module again, so there's no need to keep both around.

This seemed to be causing some scary verifier assertions in LLVM which seemed to
be uncovered by presumably (?) buggy behavior. Let's just avoid it for now and
make the wasm target slightly more lean in the process.

src/librustc_trans/back/write.rs

@@ -733,7 +733,7 @@ unsafe fn codegen(cgcx: &CodegenContext,
             // We can't use the same module for asm and binary output, because that triggers
             // various errors like invalid IR or broken binaries, so we might have to clone the
             // module to produce the asm output
-            let llmod = if config.emit_obj {
+            let llmod = if config.emit_obj && !asm2wasm {
                 llvm::LLVMCloneModule(llmod)
             } else {
                 llmod
@@ -742,7 +742,7 @@ unsafe fn codegen(cgcx: &CodegenContext,
                 write_output_file(diag_handler, tm, cpm, llmod, &path,
                                   llvm::FileType::AssemblyFile)
             })?;
-            if config.emit_obj {
+            if config.emit_obj && !asm2wasm {
                 llvm::LLVMDisposeModule(llmod);
             }
             timeline.record("asm");