reject undef/zero labels. This fixes PR3281:crash0[56].ll with these

diagnostics:
llvm-as: crash05.ll:1:14: invalid type for null constant
global label zeroinitializer addrspace (75), section "c"
             ^
llvm-as: crash06.ll:2:14: invalid type for null constant
  udiv label zeroinitializer, @0
             ^

llvm-svn: 61681