True Label and False Label of an OpBranchConditional must not be the same (#2528)

Original commit:
KhronosGroup/SPIRV-LLVM-Translator@4a6a1746c61f917

Author: vmaksimo

llvm-spirv/lib/SPIRV/libSPIRV/SPIRVInstruction.h

@@ -850,6 +850,8 @@ class SPIRVBranchConditional : public SPIRVInstruction {
            getCondition()->getType()->isTypeBool());
     assert(getTrueLabel()->isForward() || getTrueLabel()->isLabel());
     assert(getFalseLabel()->isForward() || getFalseLabel()->isLabel());
+    if (Module->isAllowedToUseVersion(VersionNumber::SPIRV_1_6))
+      assert(TrueLabelId != FalseLabelId);
   }
   SPIRVId ConditionId;
   SPIRVId TrueLabelId;