Added --compact-ids to /tools/opt

The pass remaps ids to a compact set which starts with %1 and
has no gaps.
diff --git a/include/spirv-tools/optimizer.hpp b/include/spirv-tools/optimizer.hpp
index 0b5af19..a655995 100644
--- a/include/spirv-tools/optimizer.hpp
+++ b/include/spirv-tools/optimizer.hpp
@@ -185,6 +185,10 @@
 // points are not changed.
 Optimizer::PassToken CreateInlinePass();
 
+// Creates a compact ids pass.
+// The pass remaps result ids to a compact and gapless range starting from %1.
+Optimizer::PassToken CreateCompactIdsPass();
+
 }  // namespace spvtools
 
 #endif  // SPIRV_TOOLS_OPTIMIZER_HPP_