[SelectionDAG] Add memory size for CSEMap ID calculation

In NVPTX `ReplaceVectorLoad()`, i1 and i8 types are promoted to i16,
followed by a truncate operation. Thus, v2i8 (or v2i1) and v2i16 will
have the same VTList, which causes a collision in CSEMap.

To differentiate the original VTList, let's add the size in generating
an ID. Otherwise the compiler crashes in refineAlignment:
`MMO->getSize() == getSize() && "Size mismatch!"`

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D153712