[Constants][PowerPC] Check exactlyValue for ppc_fp128 in isNullValue

 PPC_FP128 determines isZero/isNan/isInf using high-order double value
 only. Checking isZero/isNegative might return the isNullValue unexpectedly.
 eg:
   0xM0000000000000000FFFFFFFFFFFFFFFFF

isZero, but it is not NullValue.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D103634