Improve comment for ISD::EXTRACT_VECTOR_ELT

The comment in ISDOpcodes.h for EXTRACT_VECTOR_ELT now explains that the high
bits are undefined if the result is extended.

Review: Hal Finkel

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292933 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/ISDOpcodes.h

@@ -281,7 +281,8 @@ namespace ISD {
     /// EXTRACT_VECTOR_ELT(VECTOR, IDX) - Returns a single element from VECTOR
     /// identified by the (potentially variable) element number IDX.  If the
     /// return type is an integer type larger than the element type of the
-    /// vector, the result is extended to the width of the return type.
+    /// vector, the result is extended to the width of the return type. In
+    /// that case, the high bits are undefined.
     EXTRACT_VECTOR_ELT,
 
     /// CONCAT_VECTORS(VECTOR0, VECTOR1, ...) - Given a number of values of