[WebAssembly] Always use the version of computeKnownBits that returns a value. NFCI.

Continues the work started by @bogner in rL340594 to remove uses of the KnownBits output paramater version.

llvm-svn: 349911
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyInstrMemory.td b/llvm/lib/Target/WebAssembly/WebAssemblyInstrMemory.td
index ccc331d..0f8b339 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyInstrMemory.td
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyInstrMemory.td
@@ -33,10 +33,8 @@
   if (ConstantSDNode *CN = dyn_cast<ConstantSDNode>(N->getOperand(1)))
     return CurDAG->MaskedValueIsZero(N->getOperand(0), CN->getAPIntValue());
 
-  KnownBits Known0;
-  CurDAG->computeKnownBits(N->getOperand(0), Known0, 0);
-  KnownBits Known1;
-  CurDAG->computeKnownBits(N->getOperand(1), Known1, 0);
+  KnownBits Known0 = CurDAG->computeKnownBits(N->getOperand(0), 0);
+  KnownBits Known1 = CurDAG->computeKnownBits(N->getOperand(1), 0);
   return (~Known0.Zero & ~Known1.Zero) == 0;
 }]>;