[Dlang-internal] Better code gen for nothrow functions

[Walter Bright]

https://github.com/dlang/dmd/pull/7361

I'm pretty pleased with the results.
Consider the code:

   void bar();
   int test() {
     try {
         bar();
         return 0;
     } finally {
         bar();
     }
   }

This compiles to:

		push	EBP
		mov	EBP,ESP
		mov	EDX,FS:__except_list
		push	0FFFFFFFFh
		push	offset _D4test5test2FZi[087h]
		push	EDX
		mov	FS:__except_list,ESP
		sub	ESP,014h
		mov	-020h[EBP],EBX
		mov	-01Ch[EBP],ESI
		mov	-018h[EBP],EDI
		xor	EAX,EAX
		mov	-4[EBP],EAX
		call	near ptr _D4test3barFZv
		xor	EAX,EAX
		mov	dword ptr -4[EBP],0FFFFFFFFh
		push	EAX
		call	near ptr L60
		pop	EAX
		mov	ECX,-0Ch[EBP]
		mov	FS:__except_list,ECX
		mov	EBX,-020h[EBP]
		mov	ESI,-01Ch[EBP]
		mov	EDI,-018h[EBP]
		mov	ESP,EBP
		pop	EBP
		ret
		call	near ptr L60
		jmp short	L6D
L60:		mov	dword ptr -4[EBP],0FFFFFFFFh
		call	near ptr _D4test3barFZv
		ret
L6D:		mov	ECX,-0Ch[EBP]
		mov	FS:__except_list,ECX
		mov	EBX,-020h[EBP]
		mov	ESI,-01Ch[EBP]
		mov	EDI,-018h[EBP]
		mov	ESP,EBP
		pop	EBP
		ret
		mov	EAX,offset FLAT:_DATA
		jmp	near ptr __d_framehandler

But if nothrow is added to bar():

		push	EAX
		call	near ptr _D4test3barFNbZv
		call	near ptr _D4test3barFNbZv
		xor	EAX,EAX
		pop	ECX
		ret

Note that it wasn't necessary to add nothrow to test(). The disparity isn't as 
great on other platforms, but it's still significant. This will make RAII in 
-betterC much better, and nothrow functions quite worthwhile in general.