Add a textual message to the assert.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121349 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/ARM/ARMAsmPrinter.cpp b/lib/Target/ARM/ARMAsmPrinter.cpp
index 901a4ab..09124e4 100644
--- a/lib/Target/ARM/ARMAsmPrinter.cpp
+++ b/lib/Target/ARM/ARMAsmPrinter.cpp
@@ -720,7 +720,8 @@
   case ARM::t2ADDrSPi12:
   case ARM::t2SUBrSPi:
   case ARM::t2SUBrSPi12:
-    assert (MI->getOperand(1).getReg() == ARM::SP);
+    assert ((MI->getOperand(1).getReg() == ARM::SP) &&
+            "Unexpected source register!");
     break;
 
   case ARM::t2MOVi32imm: assert(0 && "Should be lowered by thumb2it pass");