Fix compiler warning when using clang 3.6.0

Without the fix we get the following (with -Werror):

../lib/Target/X86/X86ISelLowering.cpp:14181:58: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
  SmallVector<std::array<int, 2>, 2> LaneSrcs(NumLanes, {-1, -1});
                                                         ^~~~~~
                                                         {     }
1 error generated.

llvm-svn: 352455