[AMDGPU][NFC] Simplify needcopysign logic (#75176)

This was caught by coverity, reported as: `dead_error_condition`.
Since the conditional revolves around `CF`, it is guaranteed to be null
in the else clause, hence making the second part of the statement
redundant.

Author: jchlanda

llvm/lib/Target/AMDGPU/AMDGPULibCalls.cpp

@@ -1050,8 +1050,7 @@ bool AMDGPULibCalls::fold_pow(FPMathOperator *FPOp, IRBuilder<> &B,
                      CF->isNegative();
     } else {
       needlog = true;
-      needcopysign = needabs = FInfo.getId() != AMDGPULibFunc::EI_POWR &&
-                               (!CF || CF->isNegative());
+      needcopysign = needabs = FInfo.getId() != AMDGPULibFunc::EI_POWR;
     }
   } else {
     ConstantDataVector *CDV = dyn_cast<ConstantDataVector>(opr0);