compiler, runtime: optimize zero-sized allocations

Instead of referring to an unused global, use a constant value. This is
safe even when using `-gc=none` (since no actual memory gets allocated)
which wasn't the case before. It should also reduce binary size by a few
bytes for most programs.

compiler/gc.go

@@ -20,6 +20,13 @@ func (b *builder) createAlloc(sizeValue, layoutValue llvm.Value, align int, comm
 		allocFunc = "alloc_noheap"
 	}
 
+	// Allocs that don't allocate anything can return an architecture-specific
+	// sentinel value.
+	if !sizeValue.IsAConstantInt().IsNil() && sizeValue.ZExtValue() == 0 {
+		allocFunc = "alloc_zero"
+	}
+
+	// Make the runtime call.
 	call := b.createRuntimeCall(allocFunc, []llvm.Value{sizeValue, layoutValue}, comment)
 	if align != 0 {
 		// TODO: make sure all callsites set the correct alignment.