Consider b in pop_under_mask

[kammce]

‘Pop’ generally denotes removal from the stack commencing at current
vsp, with subsequent increment of vsp to beyond the removed quantities.
The sole exception to this rule is popping r13, when the writeback of
the loaded value to vsp is delayed until after the whole instruction
has completed. When multiple registers are popped by a single
instruction they are taken as lowest numbered register at lowest stack
address.

This is remark b. The code currently performs no extra work is performed if r13 is in the mask. It should follow the instructions above.