[PowerPC] Fix wrong ElemSIze when calling isConsecutiveLS()

Summary:
This issue from the bugzilla: https://bugs.llvm.org/show_bug.cgi?id=41177

When the two operands for BUILD_VECTOR are same, we will get assert error.
llvm::SDValue combineBVOfConsecutiveLoads(llvm::SDNode*, llvm::SelectionDAG&):
Assertion `!(InputsAreConsecutiveLoads && InputsAreReverseConsecutive) &&
"The loads cannot be both consecutive and reverse consecutive."' failed.

This error caused by the wrong ElemSIze when calling isConsecutiveLS(). We
should use `getScalarType().getStoreSize();` to get the ElemSize instread of
 `getScalarSizeInBits() / 8`.

Reviewed By: jsji

Differential Revision: https://reviews.llvm.org/D60811

llvm-svn: 358644