Crash after your recent commits

Hi Meador, this buildbot started failing last night and I think it was
probably you!

Here's a testcase. You can reproduce with "opt -instcombine". (The problem is
that the code isn't expecting printf to return void).

Ciao, Duncan.

@.cst = linker_private unnamed_addr constant [8 x i8] c"A::A()\0A\00", align 8

define void @main() {
   call void (i8*, ...)* @printf(i8* getelementptr inbounds ([8 x i8]* @.cst, i64 0, i64 0))
   ret void

declare void @printf(i8* nocapture, ...) nounwind

Hi Duncan,