Skip adding decorations for OpForward (#2529)

When a temporary `OpForward` instruction is needed during translation
to SPIR-V, do not add the decorations yet, as that would result in
duplicate decorations when the actual instruction is visited and the
`OpForward` is replaced by a real SPIR-V instruction.

The SPIR-V Validator has recently started checking for duplicate
decorations; this fixes some but not all issues arising from the new
checks.

Contributes to KhronosGroup/SPIRV-LLVM-Translator#2509

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@a2783135be3f37f

Author: svenvh

llvm-spirv/lib/SPIRV/SPIRVWriter.cpp

@@ -1470,7 +1470,12 @@ SPIRVValue *LLVMToSPIRVBase::transValue(Value *V, SPIRVBasicBlock *BB,
          "Invalid SPIRV BB");
 
   auto *BV = transValueWithoutDecoration(V, BB, CreateForward, FuncTrans);
-  if (!BV || !transDecoration(V, BV))
+  if (!BV)
+    return nullptr;
+  // Only translate decorations for non-forward instructions.  Forward
+  // instructions will have their decorations translated when the actual
+  // instruction is seen and rewritten to a real SPIR-V instruction.
+  if (!BV->isForward() && !transDecoration(V, BV))
     return nullptr;
   StringRef Name = V->getName();
   if (!Name.empty()) // Don't erase the name, which BM might already have