spirv-val: pointer being the Result <id> of the OpBitcast instruction

@s-perron and @alan-baker, according to the SPIR-V specification, if neither the `VariablePointers` nor `VariablePointersStorageBuffer` capabilities are declared, a pointer can be the `Result ` of only the following instructions:

- `OpVariable`
- `OpAccessChain`
- `OpInBoundsAccessChain`
- `OpFunctionParameter`
- `OpImageTexelPointer`
- `OpCopyObject`

However, currently `spirv-val` allows a pointer to be the `Result ` of the `OpBitcast` instruction, even without the `VariablePointers` and `VariablePointersStorageBuffer` capabilities.

Start at the spirv-val validation path for OpBitcast and trace how pointer result types are checked against the VariablePointers and VariablePointersStorageBuffer capabilities. Add a regression case showing that an invalid pointer result is rejected without those capabilities, then run the relevant validator tests to confirm the specified pointer-producing instructions remain accepted.

Written by the indexing model from the issue text.