[DAGCombiner] Fix the type used in canFoldInAddressingMode to account for the

right scaling.

In the function canFoldInAddressingMode, VT is computed as the type of the
destination/source of a LOAD/STORE operations, instead of the memory type of the
operation.
On targets with a scaling factor on the offset of the LOAD/STORE operations, the
function may return false for actually valid cases. This may then prevent the
selection of profitable pre or post indexed load/store operations, and instead
select pre or post indexed load/store for unprofitable cases.

Patch by Francois de Ferriere <francois.de-ferriere@st.com>!

Differential Revision: http://reviews.llvm.org/D9146

llvm-svn: 235780