[EarlyCSE][ConstantFolding] do not constant fold atan2(+/-0.0, +/-0.0)

These may raise an error (set errno) as discussed in the post-commit
comments for D127964, so we can't fold away the call and potentially
alter that behavior.

llvm/lib/Analysis/ConstantFolding.cpp

@@ -3371,10 +3371,11 @@ bool llvm::isMathLibCallNoop(const CallBase *Call,
       case LibFunc_atan2:
       case LibFunc_atan2f:
       case LibFunc_atan2l:
-        // POSIX, GLIBC and MSVC dictate atan2(0,0) is 0 and no error is raised.
-        // C11 says that a domain error may optionally occur.
-        // This is consistent with both.
-        return true;
+        // Although IEEE-754 says atan2(+/-0.0, +/-0.0) are well-defined, and
+        // GLIBC and MSVC do not appear to raise an error on those, we
+        // cannot rely on that behavior. POSIX and C11 say that a domain error
+        // may occur, so allow for that possibility.
+        return !Op0.isZero() || !Op1.isZero();
 
       default:
         break;

llvm/test/Transforms/EarlyCSE/atan.ll

@@ -50,32 +50,44 @@ define x86_fp80 @atanl_x86(x86_fp80 %x) {
   ret x86_fp80 %call
 }
 
+; This is not folded because it is known to set errno on some systems.
+
 define float @callatan2_00() {
 ; CHECK-LABEL: @callatan2_00(
+; CHECK-NEXT:    [[CALL:%.*]] = call float @atan2f(float 0.000000e+00, float 0.000000e+00)
 ; CHECK-NEXT:    ret float 0.000000e+00
 ;
   %call = call float @atan2f(float 0.0, float 0.0)
   ret float %call
 }
 
+; This is not folded because it is known to set errno on some systems.
+
 define float @callatan2_n00() {
 ; CHECK-LABEL: @callatan2_n00(
+; CHECK-NEXT:    [[CALL:%.*]] = call float @atan2f(float -0.000000e+00, float 0.000000e+00)
 ; CHECK-NEXT:    ret float -0.000000e+00
 ;
   %call = call float @atan2f(float -0.0, float 0.0)
   ret float %call
 }
 
+; This is not folded because it is known to set errno on some systems.
+
 define float @callatan2_0n0() {
 ; CHECK-LABEL: @callatan2_0n0(
+; CHECK-NEXT:    [[CALL:%.*]] = call float @atan2f(float 0.000000e+00, float -0.000000e+00)
 ; CHECK-NEXT:    ret float 0x400921FB60000000
 ;
   %call = call float @atan2f(float 0.0, float -0.0)
   ret float %call
 }
 
+; This is not folded because it is known to set errno on some systems.
+
 define float @callatan2_n0n0() {
 ; CHECK-LABEL: @callatan2_n0n0(
+; CHECK-NEXT:    [[CALL:%.*]] = call float @atan2f(float -0.000000e+00, float -0.000000e+00)
 ; CHECK-NEXT:    ret float 0xC00921FB60000000
 ;
   %call = call float @atan2f(float -0.0, float -0.0)