Sanitize debug source location tracking for implicit branch and return

This patch tries to attach debug location of a branch/return instruction to its predecessor or the closing brace. If none could be found, no debug info should be emitted.

Test/spv.debuginfo.implicit_br.glsl.frag

@@ -0,0 +1,60 @@
+#version 460
+
+out int outx;
+int counter = 0;
+
+void test_if() {
+    if (false) {
+        counter += 1;
+    }
+}
+
+void test_ifelse() {
+    if (false) {
+        counter += 1;
+    }
+    else {
+        counter += 2;
+    }
+}
+
+void test_if_compound() {
+    if (false) {
+        if (false) {
+            counter += 1;
+        }
+    }
+}
+
+void test_if_compound2() {
+    if (false) {
+        if (false) {
+            counter += 1;
+        }
+
+        counter += 2;
+    }
+}
+
+void test_switch() {
+    switch (0) {
+    case 0:
+        counter += 1;
+        // implict fallthrough
+    case 1:
+        counter += 2;
+        break;
+    default:
+        counter += 3;
+        // implicit break
+    }
+}
+
+void main() {
+    test_if();
+    test_ifelse();
+    test_if_compound();
+    test_if_compound2();
+    test_switch();
+    outx = counter;
+}