DAG: Setting Masked-Expand-Load as a variant of Masked-Load node

Masked-expand-load node represents load operation that loads a variable amount of elements from memory according to amount of "true" bits in the mask and expands the loaded elements according to their position in the mask vector.
Right now, the node is used in intrinsics for VEXPAND* instructions. 
The work is done towards implementation of masked.expandload and masked.compressstore intrinsics.

Differential Revision: https://reviews.llvm.org/D25322

llvm-svn: 283694