Remapper: remove debug info for IDs stripped in other passes

If some DCE is performed such as removing dead functions, then even
if we are NOT stripping debug info, we still must remove the debug
opcodes that refer to the now-dead IDs.

Also, this adds a small change to perform no ID remapping if none
is requested, making spirv-remap properly be a no-op if no options
are given.

SPIRV/SPVRemapper.h

@@ -239,7 +239,8 @@ private:
 
    void        applyMap();            // remap per local name map
    void        mapRemainder();        // map any IDs we haven't touched yet
-   void        stripDebug();          // strip debug info
+   void        stripDebug();          // strip all debug info
+   void        stripDeadRefs();       // strips debug info for now-dead references after DCE
    void        strip();               // remove debug symbols
    
    std::vector<spirword_t> spv;      // SPIR words
@@ -264,7 +265,6 @@ private:
    // Function start and end.  use unordered_map because we'll have
    // many fewer functions than IDs.
    std::unordered_map<spv::Id, range_t> fnPos;
-   std::unordered_map<spv::Id, range_t> fnPosDCE; // deleted functions
 
    // Which functions are called, anywhere in the module, with a call count
    std::unordered_map<spv::Id, int> fnCalls;