Executing LD1SB + MTE on Arm64 fails an assert
Description of problem:
I'm getting 
```
qemu-system-aarch64: ../tcg/tcg-op-gvec.c:91: simd_desc: Assertion `data == sextract32(data, 0, (32 - ((0 + 8) + 2)))' failed.
```
This is caused by the upper bits of `data` being set to 1, which violates the condition.
Steps to reproduce:
1. build QEMU with assertions enabled (e.g., `configure --enable-debug-tcg`).
2. have a `LD1SB f{z25.d}, p3/z, [x14, x9]` (binary a5894dd9) instruction in the executed code.
3. enable mte
4. Let QEMU execute the ld1sb instruction.
Additional information:
![image](/uploads/8b2a68b986b94549da1abf4e076c5171/image.png){width=699 height=141}

This issue happens because for ld1sb, nregs=0 in `sve.decode`:
```
# SVE contiguous load (scalar plus scalar)
LD_zprr         1010010 .... ..... 010 ... ..... .....    @rprr_load_dt nreg=0 
```
As a result, in do_mem_zpa is called with n_reg=0, which becomes mte_n inside do_mem_zpa.
Since mte_n==0, and mte_active, then 
```c
desc = FIELD_DP32(desc, MTEDESC, SIZEM1, (mte_n << msz) - 1);
```
sets (0) - 1 == -1 to the field, which also sets the upper bits of `desc`.
The `desc` with upper bits set to 1 is used to call:
```c
desc = simd_desc(vsz, vsz, zt | desc);
```
Inside `simd_desc`, the last parameter is named `data` and it fails the assertion:
```c
tcg_debug_assert(data == sextract32(data, 0, SIMD_DATA_BITS))
```

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