[MLIR] Update Vector To LLVM conversion to be aware of assume_alignment

vector.transfer_read and vector.transfer_write operations are converted
to llvm intrinsics with specific alignment information, however there
doesn't seem to be a way in llvm to take information from llvm.assume
intrinsics and change this alignment information.  In any
event, due the to the structure of the llvm.assume instrinsic, applying
this information at the llvm level is more cumbersome.  Instead, let's
generate the masked vector load and store instrinsic with the right
alignment information from MLIR in the first place.  Since
we're bothering to do this, lets just emit the proper alignment for
loads, stores, scatter, and gather ops too.

Differential Revision: https://reviews.llvm.org/D100444