Revert "Extend BasicBlock sections to allow specifying clusters of basic blocks"

This reverts commit 0d4ec16d3db3a92514e14101f635e8536c208c4f Because
tests were not added to the commit.
diff --git a/llvm/lib/CodeGen/MIRParser/MIRParser.cpp b/llvm/lib/CodeGen/MIRParser/MIRParser.cpp
index eb17434..5b6f71a 100644
--- a/llvm/lib/CodeGen/MIRParser/MIRParser.cpp
+++ b/llvm/lib/CodeGen/MIRParser/MIRParser.cpp
@@ -442,8 +442,8 @@
     MF.createBBLabels();
     MF.setBBSectionsType(BasicBlockSection::Labels);
   } else if (MF.hasBBSections()) {
+    MF.setSectionRange();
     MF.createBBLabels();
-    MF.assignBeginEndSections();
   }
   PFS.SM = &SM;