[AArch64][SVE] Pair SVE fill/spill into LDP/STP with -msve-vector-bits=128. (#134068)

When compiling with -msve-vector-bits=128 or vscale_range(1, 1) and when
the offsets allow it, we can pair SVE LDR/STR instructions into Neon
LDP/STP.

For example, given:
```cpp
#include <arm_sve.h>

void foo(double const *ldp, double *stp) {
  svbool_t pg = svptrue_b64();
  svfloat64_t ld1 = svld1_f64(pg, ldp);
  svfloat64_t ld2 = svld1_f64(pg, ldp+svcntd());
  svst1_f64(pg, stp, ld1);
  svst1_f64(pg, stp+svcntd(), ld2);
}
```

When compiled with `-msve-vector-bits=128`, we currently generate:
```gas
foo:
        ldr     z0, [x0]
        ldr     z1, [x0, #1, mul vl]
        str     z0, [x1]
        str     z1, [x1, #1, mul vl]
        ret
```

With this patch, we instead generate:
```gas
foo:
        ldp     q0, q1, [x0]
        stp     q0, q1, [x1]
        ret
```

This is an alternative, more targetted approach to #127500.