Add support for the 'O' constraint.

Patch by Jack Carter.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156285 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/Mips/MipsISelLowering.cpp b/lib/Target/Mips/MipsISelLowering.cpp
index ded6aa0..97a0ef6 100644
--- a/lib/Target/Mips/MipsISelLowering.cpp
+++ b/lib/Target/Mips/MipsISelLowering.cpp
@@ -3044,6 +3044,7 @@
   case 'K': // unsigned 16 bit immediate
   case 'L': // signed 32 bit immediate where lower 16 bits are 0
   case 'N': // immediate in the range of -65535 to -1 (inclusive)
+  case 'O': // signed 15 bit immediate (+- 16383)
     if (isa<ConstantInt>(CallOperandVal))
       weight = CW_Constant;
     break;
@@ -3146,6 +3147,16 @@
       }
     }
     return;
+  case 'O': // signed 15 bit immediate
+    if (ConstantSDNode *C = dyn_cast<ConstantSDNode>(Op)) {
+      EVT Type = Op.getValueType();
+      int64_t Val = C->getSExtValue();
+      if ((isInt<15>(Val))) {
+        Result = DAG.getTargetConstant(Val, Type);
+        break;
+      }
+    }
+    return;
   }
 
   if (Result.getNode()) {