[IRMemoryMap] Avoid redundant zero-init in the Allocation constructor (NFC)

In the lldb-bench/arithmetic benchmark, 1.7% of the total running time
is spent zero-initializing a std::vector that has already been zeroed.

llvm-svn: 339051
diff --git a/lldb/source/Expression/IRMemoryMap.cpp b/lldb/source/Expression/IRMemoryMap.cpp
index 1953e808..6c01c28 100644
--- a/lldb/source/Expression/IRMemoryMap.cpp
+++ b/lldb/source/Expression/IRMemoryMap.cpp
@@ -278,14 +278,10 @@
   default:
     assert(0 && "We cannot reach this!");
   case eAllocationPolicyHostOnly:
-    m_data.SetByteSize(size);
-    memset(m_data.GetBytes(), 0, size);
-    break;
-  case eAllocationPolicyProcessOnly:
-    break;
   case eAllocationPolicyMirror:
     m_data.SetByteSize(size);
-    memset(m_data.GetBytes(), 0, size);
+    break;
+  case eAllocationPolicyProcessOnly:
     break;
   }
 }