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
```
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