tests: add an end-to-end to check complete constant folding of generic floating point literal construction.

Author: eeckstein

test/SILOptimizer/constant_fold_float.swift

@@ -18,3 +18,40 @@ public func fold_inf_inf_cmp() -> Bool {
   0x1.0p128 < Float.infinity
 }
 
+public struct FP32: FixedPoint {
+  public var bits: Int32
+
+  public init(bits: Bits) {
+    self.bits = bits
+  }
+}
+
+// CHECK-LABEL: sil @$s4test0A19FloatLiteralFoldingAA4FP32VyF :
+// CHECK:         [[L:%.*]] = integer_literal $Builtin.Int32, 65536
+// CHECK:         [[I:%.*]] = struct $Int32 ([[L]])
+// CHECK:         [[F:%.*]] = struct $FP32 ([[I]])
+// CHECK:         return [[F]]
+// CHECK:       } // end sil function '$s4test0A19FloatLiteralFoldingAA4FP32VyF'
+public func testFloatLiteralFolding() -> FP32 {
+  return 1.0
+}
+
+public protocol FixedPoint: ExpressibleByFloatLiteral {
+  associatedtype Bits: FixedWidthInteger
+  var bits: Bits { get set }
+
+  init(bits: Bits)
+}
+
+extension FixedPoint {
+  public init(floatLiteral value: Double) {
+    self.init(value)
+  }
+
+  init<F: BinaryFloatingPoint>(_ value: F) {
+    let s = F(sign: .plus, exponent: F.Exponent(16), significand: 1)
+    let r = (s * value).rounded(.toNearestOrEven)
+    self.init(bits: Bits(exactly: r)!)
+  }
+}
+