- Disable ZeroRegZero in FP regfile (FP reg 0 does not need to be zeroed; zeroing creates issues which the compiler does not expect)
- Add lsu_qmcast_i input to FP LSU: its absence was corruptin L/S transactions from Spatz
- Fix amo assignment (purely cosmetic)

Backports from pulp-platform/spatz:

- 32d90b1a "fix a potential bug in VLSU that the index counter might be
  false triggered because of the late acceptance of an indexed store"
- 502724e8 "add conservative switch from store to load instruction"

These are required for the following fetch-logic fix to actually work:
without retirement correctly gated on outstanding writes, the newer
index-fetch-pending logic has nothing to prevent a premature retire/
re-run from racing ahead of it.

… #c919d7e3)

Backports two upstream pulp-platform/spatz fixes for indexed vector
memory ops (vsuxei/vluxei):

- 700a810f "fix next index fetch logic"
- c919d7e3 "fix index fetch logic from VRF"

The index-fetch mechanism itself was fetching correct content every
group, but the send path could pair a store with a stale
one-group-behind snapshot of the index word even though the correct
word had already been fetched (data/index pairing desync at send time,
not a fetch-timing bug).

Root cause: vrf_re_o[intf][1] = mem_is_indexed was asserted
unconditionally for the whole duration of an indexed instruction,
regardless of whether that particular interface still had elements
left to process. Once one interface finished, it kept re-requesting
the shared index-VRF read port every cycle for no reason, permanently
winning arbitration against the other interface's still-needed
request and starving it of vrf_rvalid_i forever.

Fix: gate the index-read request on |mem_operation_valid[intf] too, so
a finished interface stops contending for the port. The data-read port
(vrf_re_o[intf][0], used by stores) was already correctly gated on
commit_operation_valid[intf].

For VFU/VSLDU write ports, done_result_d[intID] is latched sticky once
vl_cnt crosses a completion threshold (spatz_controller.sv, the
"VFU and VSLDU" branch). For VLSU write ports (SB_VLSU_VD_WD0/WD1),
the mirrored "VLSU: intID is fixed per interface" branch sets
wrote_result_d (a single-cycle pulse tied directly to the write) but
never sets done_result_d at all.
A chained consumer's read gate can then only ever pass by having its
read evaluated on the exact same cycle the producer's wrote_result_q
pulses; if that doesn't line up (as with a tiny vl=4 unit-stride load,
which can complete in a single beat on one interface), the read either
stalls forever or gets granted against unwritten/stale VRF read data,
propagating X into the consumer.

Fix: add a per-interface completion counter (vlsu_vl_cnt_q/d) tracking
each VLSU interface's own write progress independently (interfaces
write in parallel under double_bw, unlike VFU/VSLDU's single
sequential vl_cnt), and latch done_result_d sticky once an interface's
share is fully written. Written as (cnt + size >= max) rather than
(cnt >= max - size) since vl_max can be smaller than a single
VRFWordBWidth write for small vl, which would underflow the subtracted
form.