Validator support for Variable Pointer extension.

Validator support for Variable Pointer extension.

If the variable_pointer extension is used:

* OpLoad's pointer argument may be the result of any of the following:
 * OpSelect
 * OpPhi
 * OpFunctionCall
 * OpPtrAccessChain
 * OpCopyObject
 * OpLoad
 * OpConstantNull

* Return value of a function may be a pointer.

* It is valid to use a pointer as the return value of a function.

* OpStore should allow a variable pointer argument.