Introduce enum values for previously defined metadata types. (NFC)

Our metadata scheme lazily assigns IDs to string metadata, but we have a mechanism to preassign them as well.  Using a preassigned ID is helpful since we get compile time type checking, and avoid some (minimal) string construction and comparison.  This change adds enum value for three existing metadata types:
+    MD_nontemporal = 9, // "nontemporal"
+    MD_mem_parallel_loop_access = 10, // "llvm.mem.parallel_loop_access"
+    MD_nonnull = 11 // "nonnull"

I went through an updated various uses as well.  I made no attempt to get all uses; I focused on the ones which were easily grepable and easily to translate.  For example, there were several items in LoopInfo.cpp I chose not to update.

llvm-svn: 220248
diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index 4952a64..5f00bb9 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -444,7 +444,7 @@
     // in the same alias set as something that ends up being modified.
     if (AA->pointsToConstantMemory(LI->getOperand(0)))
       return true;
-    if (LI->getMetadata("invariant.load"))
+    if (LI->getMetadata(LLVMContext::MD_invariant_load))
       return true;
 
     // Don't hoist loads which have may-aliased stores in loop.