[AArch64][SVE] Fix range for DUP immediates (16bit elts)

For immediates used in DUP instructions that have the range
-128 to 127, or a multiple of 256 in the range -32768 to 32512,
one could argue that when the result element size is 16bits (.h),
the value can be considered both signed and unsigned.

Reviewers: rengolin, fhahn, SjoerdMeijer, samparker, javed.absar

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D47619

llvm-svn: 333873