[XCore] Update disassembler test to improve coverage of the instructions.

Previously some instructions were unintentionally covered twice and
others were not covered at all.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178688 91177308-0d34-0410-b5e6-96231b3b80d8
[XCore] Check disassembly of the st8 instruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178689 91177308-0d34-0410-b5e6-96231b3b80d8