Commit b84bf58a accidentally extended the range of allowed negative
numbers.
* config/tc-ppc.c (ppc_insert_operand): Trim PPC_OPERAND_SIGNOPT
allowed negative range.
* testsuite/gas/ppc/power9.s: Test xxspltib of -128, not -256.
* testsuite/gas/ppc/power9.d: Update.