[SVE] Fix invalid assert in expand_DestructiveOp.

AArch64ExpandPseudo::expand_DestructiveOp contains an assert to
ensure the destructive operand's register is unique.  However,
this is only required when psuedo expansion emits a movprfx.

A simple example when a movprfx is not required is
  Z0 = FADD_ZPZZ_UNDEF_S P0, Z0, Z0
which expands to an unprefixed FADD_ZPmZ_S instruction.

This patch moves the assert to the places where a movprfx is emitted.

Differential Revision: https://reviews.llvm.org/D83029