fix(CI): long-running emulated rv64 matmul integration tests (#24593)

The RISCV-64 e2e matmul integration tests without the ukernels are very
long-running under QEMU emulation, and were causing timeouts in CI. This
PR disables the generic (non-uk) vectorization path for those tests
until we properly support codegen.

(Claude-generated summary)

## Summary

The RISC-V 64 e2e matmul integration tests that run **without ukernels**
(`--iree-opt-data-tiling --iree-llvmcpu-enable-ukernels=none`) are
extremely
slow under QEMU and were timing out CI (60 s per-test limit). This PR
adds
`noriscv` to all non-ukernel `dt`/`experimental_dt` matmul variants,
since the
generic vectorization path for data-tiled matmul is not yet properly
supported
on RISC-V — only the ukernel path is validated there.

**Why it's slow:** with data-tiling on but ukernels off, the packed
`mmt4d` is
lowered through generic vectorization. For the wide `M0×N0` tile it
selects
(`N0 = VLEN/8`, `M0 = 7`), the widening multiply-accumulate has no
vector×scalar
form, so the inner loop rebuilds the LHS broadcast every K-step via a
storm of
`vsetvli` reconfigurations and `vrgather`/`vslideup` permutes — e.g. for
i8 at
VLEN=256, ~118 `vsetvli` + ~19 `vrgather` per dispatch, vs ~2 `vsetvli`
and no
gathers for the ukernel. QEMU emulates every vector instruction
element-by-element, and `vsetvli`/`vrgather`/`vslide` are among the
costliest to
emulate, so the full shape set runs for tens of seconds and trips the
timeout.
`dt_i8_i32` and `experimental_dt_i8_i32` hit it first; `dt_f32_f32` was
already
at ~52 s (one slow runner from flaking).

Failing CI run:
https://github.com/iree-org/iree/actions/runs/27192246864/job/80276315195

**Emulation vs. real hardware:** the timeout is largely a QEMU artifact.
On a
SpaceMiT X60 (VLEN=256, native), the heaviest shape (540×332×516) runs
in
milliseconds — but the generic path is still 3.5–6× slower than the
ukernel
path, confirming it exercises an unsupported/unoptimized path rather
than just
an emulator quirk:

| variant (540×332×516) | 1 thread | 8 threads | generic ÷ ukernel |
|---|---|---|---|
| i8 generic  | 123 ms | 37.9 ms | **6.1×** |
| i8 ukernel  | 20.3 ms | 6.15 ms | — |
| f32 generic | 108 ms | 29.1 ms | **3.5×** |
| f32 ukernel | 30.9 ms | 9.08 ms | — |

(The gap is specific to the wide-`M0` tile; narrow-M shapes pick `M0=1`,
emit
clean `vwmacc.vx`, and are ~1.3×.)


Assisted-by: Claude Code

---------

Signed-off-by: Ege Beysel <beyselege@gmail.com>