[mlir][StandardToSPIRV] Emulate bitwidths not supported for store op.

[mlir][StandardToSPIRV] Emulate bitwidths not supported for store op.

Summary:
As D78974, this patch implements the emulation for store op. The emulation is
done with atomic operations. E.g., if the storing value is i8, rewrite the
StoreOp to:

 1) load a 32-bit integer
 2) clear 8 bits in the loading value
 3) store 32-bit value back
 4) load a 32-bit integer
 5) modify 8 bits in the loading value
 6) store 32-bit value back

The step 1 to step 3 are done by AtomicAnd as one atomic step, and the step 4
to step 6 are done by AtomicOr as another atomic step.

Differential Revision: https://reviews.llvm.org/D79272