Remove SPV_OPERAND_TYPE_VARIABLE_MEMORY_ACCESS

Remove SPV_OPERAND_TYPE_VARIABLE_MEMORY_ACCESS

If a memory mask operand is present, it is a mask.  The mask appears
only once, so just use SPV_OPERAND_TYPE_OPTIONAL_MEMORY_MASK.

The "variable literals" aspect comes into play as follows: if the
Aligned bit is set in the mask, then the parser will be made to
expect the alignment value as a literal number operand that follows
the mask.  That is done through mask operand expansion.