Stop zero-initializing memory used to store return values.

This seems to have a noticable (small) performance benefit on a
few of the example workloads.

ctx.cpp

@@ -160,8 +160,6 @@ FunctionEmitContext::FunctionEmitContext(Function *func, Symbol *funSym,
     else {
         LLVM_TYPE_CONST llvm::Type *ftype = returnType->LLVMType(g->ctx);
         returnValuePtr = AllocaInst(ftype, "return_value_memory");
-        // FIXME: don't do this store???
-        StoreInst(llvm::Constant::getNullValue(ftype), returnValuePtr);
     }
 
     if (m->diBuilder) {