[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

llvm/lib/IR/Constants.cpp

@@ -90,7 +90,9 @@ bool Constant::isNullValue() const {
 
   // +0.0 is null.
   if (const ConstantFP *CFP = dyn_cast<ConstantFP>(this))
-    return CFP->isZero() && !CFP->isNegative();
+    // ppc_fp128 determine isZero using high order double only
+    // Should check the bitwise value to make sure all bits are zero.
+    return CFP->isExactlyValue(+0.0);
 
   // constant zero is zero for aggregates, cpnull is null for pointers, none for
   // tokens.

llvm/test/CodeGen/PowerPC/ppc_fp128-bcwriter.ll

@@ -1,7 +1,7 @@
 ; RUN: llvm-as < %s -o - | llvm-dis - | FileCheck %s
 
 ;CHECK-LABEL: main
-;CHECK: store ppc_fp128 0xM0000000000000000000000000000000
+;CHECK: store ppc_fp128 0xM0000000000000000FFFFFFFFFFFFFFFF
 
 define i32 @main() local_unnamed_addr {
 _main_entry: