[AArch64][SVE] Add the @llvm.aarch64.sve.dup.x intrinsic

Summary:
This intrinsic implements the unpredicated duplication of scalar values
and is mapped to (through ISD::SPLAT_VECTOR):
  * DUP <Zd>.<T>, #<imm>
  * DUP <Zd>.<T>, <R><n|SP>

Reviewed by: sdesmalen

Differential Revision: https://reviews.llvm.org/D75900