Allow specific optimization passes to be requested from the command line.

Now you can run "`ldc test.d -c -mem2reg -simplifycfg`" if you feel the urge.
The -O<N> options are still supported, and are inserted in the passes list in
the position where they appear on the command line.
(so -simplifycfg -O1 -instcombine does the "right thing")

One small change: -inline is renamed to -enable-inlining due to a naming
conflict with the option to add the -inline pass. -inline now inserts the
inlining pass in the position specified, not in the middle of -O<N>.
(ldmd has been updated to translate -inline to -enable-inlining)

gen/linker.cpp

@@ -13,6 +13,7 @@
 #define NO_COUT_LOGGER
 #include "gen/logger.h"
 #include "gen/cl_options.h"
+#include "gen/optimizer.h"
 
 //////////////////////////////////////////////////////////////////////////////
 
@@ -111,12 +112,12 @@ int linkExecutable(const char* argv0)
         args.push_back("-strip-debug");
 
     // optimization level
-    if (!global.params.optimize)
+    if (!optimize())
         args.push_back("-disable-opt");
     else
     {
         const char* s = 0;
-        switch(global.params.optimizeLevel)
+        switch(optLevel())
         {
         case 0:
             args.push_back("-disable-opt");
@@ -138,7 +139,7 @@ int linkExecutable(const char* argv0)
     }
 
     // inlining
-    if (!(global.params.useInline || global.params.llvmInline))
+    if (!(global.params.useInline || doInline()))
     {
         args.push_back("-disable-inlining");
     }