cmd/compile: use desired register only if it satisfies register mask
In the register allocator, if possible, we allocate a value to its
desired register (the ideal register for its next use). In some
cases the desired register does not satisfies the value's output
register mask. We should not use the register in this case.
In the following example, v33 is going to be returned as a
function result, so it is allocated to its desired register AX.
However, its Op cannot use AX as output, causing miscompilation.