[Profiler] Avoid introducing empty unreachable regions

When computing the counter for the region
following a labeled statement such as `if`, avoid
adding a new empty region if the counter for such
a region is known to be zero, i.e unreachable.
This avoids adding spurious unreachable regions
after an if statements where each branch returns,
throws, or otherwise jumps to a parent statement.

We will however still add the region if any code
follows such a statement, making it non-empty.

rdar://29390569

test/Profiler/coverage_if.swift

@@ -17,3 +17,74 @@ func foo(x : Bool) { // CHECK: [[@LINE]]:20 -> {{[0-9]+}}:2 : 0
 
 foo(x: true);
 foo(x: false);
+
+// rdar://29390569 – Make sure we don't add a spurious unreachable empty region.
+
+// CHECK-LABEL: sil_coverage_map {{.*}}// coverage_if.foo1
+func foo1() -> Int {    // CHECK: [[@LINE]]:20 -> {{[0-9]+}}:2 : 0
+  if .random() {        // CHECK: [[@LINE]]:6 -> [[@LINE]]:15 : 0
+    return 0            // CHECK: [[@LINE-1]]:16 -> [[@LINE+2]]:4 : 1
+                        // CHECK: [[@LINE+1]]:4 -> {{[0-9]+}}:2 : (0 - 1)
+  } else if .random() { // CHECK: [[@LINE]]:13 -> [[@LINE]]:22 : (0 - 1)
+    return 1            // CHECK: [[@LINE-1]]:23 -> [[@LINE+1]]:4 : 2
+  } else {              // CHECK: [[@LINE]]:4 -> {{[0-9]+}}:2 : ((0 - 1) - 2)
+    return 2            // CHECK: [[@LINE-1]]:10 -> [[@LINE+1]]:4 : ((0 - 1) - 2)
+  }                     // CHECK-NOT: zero
+}
+
+// ...but we will add an unreachable region if you write code there.
+// CHECK-LABEL: sil_coverage_map {{.*}}// coverage_if.foo2
+func foo2() -> Int {
+  if .random() {
+    return 0
+  } else if .random() {
+    return 1
+  } else {
+    return 2
+  }
+  _ = foo1() // CHECK: [[@LINE]]:3 -> [[@LINE+1]]:2 : zero
+}
+
+// Make sure we don't add unreachable regions for a labeled jump either.
+// CHECK-LABEL: sil_coverage_map {{.*}}// coverage_if.foo3
+func foo3() -> Int { // CHECK: [[@LINE]]:20 -> {{[0-9]+}}:2 : 0
+  x: do {            // CHECK: [[@LINE]]:9 -> [[@LINE+6]]:4 : 0
+    if .random() {   // CHECK: [[@LINE]]:8 -> [[@LINE]]:17 : 0
+      return 0       // CHECK: [[@LINE-1]]:18 -> [[@LINE+1]]:6 : 1
+    } else {         // CHECK: [[@LINE]]:6 -> [[@LINE+3]]:4 : (0 - 1)
+      break x        // CHECK: [[@LINE-1]]:12 -> [[@LINE+1]]:6 : (0 - 1)
+    }                // CHECK-NOT: zero
+  }
+  return 2
+}
+// CHECK-LABEL: sil_coverage_map {{.*}}// coverage_if.foo4
+func foo4() -> Int {
+  x: do {
+    if .random() {
+      return 0
+    } else {
+      break x
+    }
+    let y = 0 // CHECK: [[@LINE]]:13 -> [[@LINE+1]]:4 : zero
+  }
+  return 2
+}
+
+// CHECK-LABEL: sil_coverage_map {{.*}}// coverage_if.foo5
+func foo5() throws -> Int { // CHECK: [[@LINE]]:27 -> {{[0-9]+}}:2 : 0
+  if .random() {            // CHECK: [[@LINE]]:6 -> [[@LINE]]:15 : 0
+    return 0                // CHECK: [[@LINE-1]]:16 -> [[@LINE+1]]:4 : 1
+  } else {                  // CHECK: [[@LINE]]:4 -> {{[0-9]+}}:2 : (0 - 1)
+    struct S: Error {}      // CHECK: [[@LINE-1]]:10 -> [[@LINE+2]]:4 : (0 - 1)
+    throw S()
+  }                         // CHECK-NOT: zero
+}
+
+// CHECK-LABEL: sil_coverage_map {{.*}}// coverage_if.foo6
+func foo6(_ x: Int?) -> Int? { // CHECK: [[@LINE]]:30 -> {{[0-9]+}}:2 : 0
+  if let x = x {
+    return x                   // CHECK: [[@LINE-1]]:16 -> [[@LINE+1]]:4 : 1
+  } else {                     // CHECK: [[@LINE]]:4 -> {{[0-9]+}}:2 : (0 - 1)
+    return nil                 // CHECK: [[@LINE-1]]:10 -> [[@LINE+1]]:4 : (0 - 1)
+  }                            // CHECK-NOT: zero
+}