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

This reverts commit 29a50c58.

After LLVM lowering, the original patch incorrectly moved alignment
information across an unconstrained GEP operation.  This is only correct
for some index offsets in the GEP.  It seems that the best approach is,
in fact, to rely on LLVM to propagate information from the llvm.assume()
to users.

Thanks to Thomas Raoux for catching this.