[MemOperands] Clarify code around dropping memory operands [NFC]

Clarify a comment about what it means to drop memory operands from an instruction.  While I'm adding change the name of the method slightly to make it a bit more clear what's going on when reading calling code.

llvm-svn: 256346
diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index e41926a..604feedd 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -793,7 +793,7 @@
 
     if (MBBICommon->mayLoad() || MBBICommon->mayStore())
       if (!hasIdenticalMMOs(&*MBBI, &*MBBICommon))
-        MBBICommon->clearMemRefs();
+        MBBICommon->dropMemRefs();
 
     ++MBBI;
     ++MBBICommon;