[mlir][llvm] Add support for importing masked intrinsics from LLVM IR.

The revision adds support for importing the masked load/store and
gather/scatter intrinsics from LLVM IR. To enable the import, the
revision also includes an extension of the mlirBuilder code generation
to support variadic arguments.

Depends on D136057

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D136058