[mlir][spirv] Support OpenCL when lowering memref load/store

-Add awareness to Kernel vs Shader capability for memref to SPIR-V
 lowering.
-Add lowering using spv.PtrAccessChain for Kernel capability.
-Enable lowering from scalar pointee types for kernel capabilities.

Reviewed By: antiagainst

Differential Revision: https://reviews.llvm.org/D132714