[benchmark] Add another test to floating point conversion benchmark

Author: xwu

benchmark/single-source/FloatingPointConversion.swift

@@ -18,6 +18,11 @@ public let FloatingPointConversion = [
     runFunction: run_ConvertFloatingPoint_MockFloat64ToDouble,
     tags: [.validation, .api],
     setUpFunction: { blackHole(mockFloat64s) }),
+  BenchmarkInfo(
+    name: "ConvertFloatingPoint.MockFloat64Exactly",
+    runFunction: run_ConvertFloatingPoint_MockFloat64Exactly,
+    tags: [.validation, .api],
+    setUpFunction: { blackHole(mockFloat64s) }),
   BenchmarkInfo(
     name: "ConvertFloatingPoint.MockFloat64ToInt64",
     runFunction: run_ConvertFloatingPoint_MockFloat64ToInt64,
@@ -147,20 +152,30 @@ let doubles = [
 
 let mockFloat64s = doubles.map { MockFloat64($0) }
 
+// See also: test/SILOptimizer/floating_point_conversion.swift
+
+@inline(never)
+public func run_ConvertFloatingPoint_MockFloat64ToDouble(_ N: Int) {
+  for _ in 0..<(N * 100) {
+    for element in mockFloat64s {
+      let f = Double(identity(element))
+      blackHole(f)
+    }
+  }
+}
+
 @inline(__always)
 func convert<
   T: BinaryFloatingPoint, U: BinaryFloatingPoint
->(_ value: T, to: U.Type) -> U {
-  U(value)
+>(exactly value: T, to: U.Type) -> U? {
+  U(exactly: value)
 }
 
-// See also: test/SILOptimizer/floating_point_conversion.swift
-
 @inline(never)
-public func run_ConvertFloatingPoint_MockFloat64ToDouble(_ N: Int) {
+public func run_ConvertFloatingPoint_MockFloat64Exactly(_ N: Int) {
   for _ in 0..<(N * 100) {
     for element in mockFloat64s {
-      let f = Double(identity(element))
+      let f = convert(exactly: identity(element), to: Double.self)
       blackHole(f)
     }
   }