[mlir][spirv] Add control for non-32-bit scalar type emulation

Non-32-bit scalar types require special hardware support that may not
exist on all GPUs. This is reflected in SPIR-V as that non-32-bit scalar
types require special capabilities or extensions.

Previously when there is a non-32-bit type and no native support, we
unconditionally emulate it with 32-bit ones. This isn't good given that
it can have implications over ABI and data layout consistency.

This commit introduces an option to control whether to use 32-bit
types to emulate.

Differential Revision: https://reviews.llvm.org/D100059