GlobalISel: Add comment to clarify G_BUILD_VECTOR

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351428 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/llvm/Target/GenericOpcodes.td b/include/llvm/Target/GenericOpcodes.td
index 045fe25..4a27ad7 100644
--- a/include/llvm/Target/GenericOpcodes.td
+++ b/include/llvm/Target/GenericOpcodes.td
@@ -689,7 +689,9 @@
   let hasSideEffects = 0;
 }
 
-/// Create a vector from multiple scalar registers.
+/// Create a vector from multiple scalar registers. No implicit
+/// conversion is performed (i.e. the result element type must be the
+/// same as all source operands)
 def G_BUILD_VECTOR : GenericInstruction {
   let OutOperandList = (outs type0:$dst);
   let InOperandList = (ins type1:$src0, variable_ops);