[AArch64][SVE] Avoid movprfx by reusing register for _UNDEF pseudos. (#166926)

For predicated SVE instructions where we know that the inactive lanes
are undef, it is better to pick a destination register that is not
unique. This avoids introducing a movprfx to copy a unique register to
the destination operand, which would be needed to comply with the tied
operand constraints.

For example:
```
  %src1 = COPY $z1
  %src2 = COPY $z2
  %dst = SDIV_ZPZZ_S_UNDEF %p, %src1, %src2
```
Here it is beneficial to pick $z1 or $z2 as the destination register,
because if it would have chosen a unique register (e.g. $z0) then the
pseudo expand pass would need to insert a MOVPRFX to expand the
operation into:
```
  $z0 = SDIV_ZPZZ_S_UNDEF $p0, $z1, $z2

  ->

  $z0 = MOVPRFX $z1
  $z0 = SDIV_ZPmZ_S $p0, $z0, $z2
```
By picking $z1 directly, we'd get:
```
  $z1 = SDIV_ZPmZ_S, $p0 $z1, $z2
```