[TTI] fixing a bug in the isLegalMaskedScatter API

[magabari]

Summary

isLegalMaskedScatter called the Gather version which is a bug.
use test case is provided within the review of AVX2 gathers at: https://reviews.llvm.org/D35772

[RKSimon]

LGTM - thanks

llvm/trunk/lib/Analysis/TargetTransformInfo.cpp

bool TargetTransformInfo::isLegalMaskedLoad(Type *DataType) const {
   return TTIImpl->isLegalMaskedLoad(DataType);
 }
 
 bool TargetTransformInfo::isLegalMaskedGather(Type *DataType) const {
   return TTIImpl->isLegalMaskedGather(DataType);
 }
 
 bool TargetTransformInfo::isLegalMaskedScatter(Type *DataType) const {
-  return TTIImpl->isLegalMaskedGather(DataType);
+  return TTIImpl->isLegalMaskedScatter(DataType);
 }
 
 bool TargetTransformInfo::prefersVectorizedAddressing() const {
   return TTIImpl->prefersVectorizedAddressing();
 }
 
 int TargetTransformInfo::getScalingFactorCost(Type *Ty, GlobalValue *BaseGV,
                                               int64_t BaseOffset,