Convert MaskedValueIsZero and all its users to use APInt. Also add
a SignBitIsZero function to simplify a common use case.

llvm-svn: 47561
diff --git a/llvm/lib/Target/Alpha/AlphaISelDAGToDAG.cpp b/llvm/lib/Target/Alpha/AlphaISelDAGToDAG.cpp
index d1e1311..b0ca39b 100644
--- a/llvm/lib/Target/Alpha/AlphaISelDAGToDAG.cpp
+++ b/llvm/lib/Target/Alpha/AlphaISelDAGToDAG.cpp
@@ -91,7 +91,9 @@
       // see if the missing bits (0x1000) are already known zero if not, the zap
       // isn't okay to do, as it won't clear all the required bits.
       if (BitsToCheck &&
-          !CurDAG->MaskedValueIsZero(LHS, BitsToCheck))
+          !CurDAG->MaskedValueIsZero(LHS,
+                                     APInt(LHS.getValueSizeInBits(),
+                                           BitsToCheck)))
         return 0;
       
       return Result;