This was done because the implementation depends on the value of PC
(it's a jump-to-next), and PC-dependent instructions are permitted to
be flagged as invalid in debug mode, to permit sharing of PC and the
dpc CSR.
However this is not valid in this case because the dependency on PC is
an implementation detail, not an architected dependency. Instead just
suppress the jump in debug mode. Suppressing the jump is still required
to avoid flushing following program buffer entries from the prefetch
queue during debug mode execution.
From a functional point of view not much has changed, it just removes
an inconsistency where fence.i appeared to be implemented in M/U mode
but not in debug mode. This removes a complaint from openocd when it
executes a fence + fence.i after writing to memory.
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