set noreturn attribute on finish-rewriting call site (#1087)

This is one of a sequence of PRs designed to make progress towards
generating stack maps so that we can trigger the GC during allocation
rather than in between rewrite steps only. The first few PRs will be
preliminaries that add small features that will be used by future PRs.

In this PR we simply add the `noreturn` attribute to the call site of
the `finish_rewriting` function so that llvm is able to statically
understand that the `unreachable` instruction after the call site is in
fact unreachable.

Author: Dwight Guth

lib/codegen/Decision.cpp

@@ -863,11 +863,12 @@ void abort_when_stuck(
     }
     ptr = new llvm::BitCastInst(block, block_ptr, "", current_block);
   }
+  auto *func = get_or_insert_function(
+      module, "finish_rewriting", llvm::Type::getVoidTy(ctx), block_ptr,
+      llvm::Type::getInt1Ty(ctx));
+  func->setDoesNotReturn();
   llvm::CallInst::Create(
-      get_or_insert_function(
-          module, "finish_rewriting", llvm::Type::getVoidTy(ctx), block_ptr,
-          llvm::Type::getInt1Ty(ctx)),
-      {ptr, llvm::ConstantInt::getTrue(ctx)}, "", current_block);
+      func, {ptr, llvm::ConstantInt::getTrue(ctx)}, "", current_block);
   new llvm::UnreachableInst(ctx, current_block);
 }