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.

gtests/Spv.FromFile.cpp

@@ -956,6 +956,7 @@ INSTANTIATE_TEST_SUITE_P(
         "spv.debuginfo.rt_types.glsl.rgen",
         "spv.debuginfo.include.glsl.frag",
         "spv.debuginfo.multiline.glsl.frag",
+        "spv.debuginfo.implicit_br.glsl.frag",
     })),
     FileNameAsCustomTestSuffix
 );