x86 NCG: fix regUsageOfInstr for VMOVU & friends

This MR fixes the implementation of regUsageOfInstr for vector operations that take an Operand as the destination, by ensuring that when the destination is an address then the address should be READ, and not WRITTEN.

Getting this wrong was a disaster, as it meant the register allocator had incorrect information, which can lead to it discarding stores to registers, segfaults ensuing.