[llvm] [KnownFPClass] Refine known classes for `atan2(y, x)` when `x` is negative (PR #214629)

[via llvm-commits]

================
@@ -1980,6 +1980,27 @@ TEST_F(ComputeKnownFPClassTest, PowiInfSecond) {
   expectKnownFPClass(fcAllFlags, std::nullopt, A7);
 }
 
+TEST_F(ComputeKnownFPClassTest, Atan2DemandXSign) {
+  parseAssembly("declare float @llvm.atan2.f32(float, float)\n"
+                "declare float @llvm.fabs.f32(float)\n"
+                "define float @test(float %y, float %x) #0 {\n"
+                "  %abs = call float @llvm.fabs.f32(float %x)\n"
+                "  %sum = fadd float %abs, 1.0\n"
+                "  %negative = fneg float %sum\n"
+                "  %A = call float @llvm.atan2.f32(float %y, float %negative)\n"
+                "  ret float %A\n"
+                "}\n"
+                "attributes #0 = { \"denormal-fp-math\"=\"ieee,ieee\" }\n");
+  // atan2(y, -(|x| + 1.0))
+  // Note that -(|x| + 1.0) is never positive, zero, or subnormal.
+  // atan2(y, negative_x) is never zero or subnormal. But we need to know that
+  // x is never positive or a negative subnormal to make this deduction. Which
+  // requires us to pass more than just InterestedClasses, which is the purpose
+  // of this test.
+  KnownFPClass Known = computeKnownFPClass(A, M->getDataLayout(), fcPosZero);
+  EXPECT_EQ(fcNan | fcNormal, Known.KnownFPClasses);
+}
+
----------------
ZERICO2005 wrote:

I was able to come up with this test to justify the existence of this code:
```c++
      if ((InterestedClasses & (fcZero | fcSubnormal)) != fcNone)
        InterestedX |= fcPositive | fcNegSubnormal;
```

https://github.com/llvm/llvm-project/pull/214629