Fix return type of LongBinaryOp<Power>

Summary:
It will return a float when the exponent is negative.  Caught when running tests
with AutoJIT + type profiling.

I don't see other instances of `Float | Long` types in HIR anywhere yet, but it
appears to work fine.

Reviewed By: mpage, swtaarrs

Differential Revision: D50091025

fbshipit-source-id: 77ad3b50a48da9c28bf43f776a163c88e068d8f3

Jit/hir/ssa.cpp

@@ -517,6 +517,10 @@ Type outputType(
       if (binop.op() == BinaryOpKind::kTrueDivide) {
         return TFloatExact;
       }
+      if (binop.op() == BinaryOpKind::kPower) {
+        // Will be floating-point for negative exponents.
+        return TFloatExact | TLongExact;
+      }
       return TLongExact;
     }
     case Opcode::kLongCompare:

RuntimeTests/hir_tests/profile_data_hir_test.txt

@@ -308,6 +308,75 @@ fun jittestmodule:test {
   }
 }
 ---
+SpecializePowerHasCorrectType
+---
+class Point:
+  def __init__(self, x, y):
+    self.x = x
+    self.y = y
+
+  __slots__ = ("x", "y")
+
+def test(p):
+  return p.x ** p.y
+
+test(Point(1, -2))
+---
+fun jittestmodule:test {
+  bb 0 {
+    v4:Object = LoadArg<0; "p">
+    Snapshot
+    v6:ObjectUser[Point:Exact] = GuardType<ObjectUser[Point:Exact]> v4 {
+      GuiltyReg v4
+    }
+    DeoptPatchpoint<0xdeadbeef> {
+      Descr 'member descriptor attribute'
+      GuiltyReg v6
+    }
+    UseType<ObjectUser[Point:Exact]> v6
+    v14:OptObject = LoadField<x@16, OptObject, borrowed> v6
+    v15:Object = CheckField<"x"> v14 {
+      GuiltyReg v6
+      FrameState {
+        NextInstrOffset 4
+        Locals<1> v6
+      }
+    }
+    Snapshot
+    DeoptPatchpoint<0xdeadbeef> {
+      Descr 'member descriptor attribute'
+      GuiltyReg v6
+    }
+    UseType<ObjectUser[Point:Exact]> v6
+    v16:OptObject = LoadField<y@24, OptObject, borrowed> v6
+    v17:Object = CheckField<"y"> v16 {
+      GuiltyReg v6
+      FrameState {
+        NextInstrOffset 8
+        Locals<1> v6
+        Stack<1> v15
+      }
+    }
+    Snapshot
+    v11:LongExact = GuardType<LongExact> v15 {
+      GuiltyReg v15
+    }
+    v12:LongExact = GuardType<LongExact> v17 {
+      GuiltyReg v17
+    }
+    UseType<LongExact> v11
+    UseType<LongExact> v12
+    v18:{FloatExact|LongExact} = LongBinaryOp<Power> v11 v12 {
+      FrameState {
+        NextInstrOffset 10
+        Locals<1> v6
+      }
+    }
+    Snapshot
+    Return v18
+  }
+}
+---
 SpecializeSplitDictLoadAttr
 ---
 class Person: