Tell the NCG that XOR foo, foo does *not* read foo

Tell the NCG that XOR foo, foo does *not* read foo

On x86[-64], MachCodeGen uses the old XOR trick to zero a register.
This patch makes regUsage not list the register as an input in this
case.
Listing the register as an input for the instruction could make it
appear like an unitialised value, which is bad because unitialised
values can cause the register allocator to panic (at least in the
presence of a loop).