[Patch][Clang/CodeGen] Fixes invalid shufflevector instruction

[Pflanzer, Moritz]

Ping.

> On 18 Jun 2015, at 22:52, Moritz Pflanzer <moritz at pflanzer14@imperial.ac.uk> wrote:
> 
> Hello,
> 
> The attached patch fixes bug 23800 ( https://llvm.org/bugs/show_bug.cgi?id=23800 ).
> There existed a case where the index operand from extractelement was directly used to create a shufflevector mask. Since the index can be of any integral type but the mask must only contain 32 bit integers a 64 bit index operand led to an assertion error later on.
> 
> I would appreciate if someone could review and eventually apply my changes.
> 
> Best regards,
> 
> Moritz
> 
> <VisitInitListExpr.patch>