[X86] Add skeleton support for icelake CPU.

There are several patches out for review right now to implement Icelake features. This adds a CPU to collect them under.

llvm-svn: 318612
diff --git a/llvm/lib/Target/X86/X86Subtarget.h b/llvm/lib/Target/X86/X86Subtarget.h
index bd164e7..56587bc 100644
--- a/llvm/lib/Target/X86/X86Subtarget.h
+++ b/llvm/lib/Target/X86/X86Subtarget.h
@@ -70,7 +70,8 @@
     IntelSkylake,
     IntelKNL,
     IntelSKX,
-    IntelCannonlake
+    IntelCannonlake,
+    IntelIcelake,
   };
 
   /// X86 processor family: Intel Atom, and others