[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