Add a field to SILOption to represent the current optimization level.

The value is set based on the -O command-line option. It is generally useful if SIL optimization passes can check the current optimization level.
A couple of subsequent commits are going to make use of this information.

Swift SVN r29653

include/swift/AST/SILOptions.h

@@ -42,6 +42,15 @@ public:
     LinkAll
   };
 
+  /// Representation of optimization modes.
+  enum class SILOptMode: unsigned {
+    NotSet,
+    None,
+    Debug,
+    Optimize,
+    OptimizeUnchecked
+  };
+
   /// Controls how  perform SIL linking.
   LinkingMode LinkMode = LinkNormal;
 
@@ -57,6 +66,9 @@ public:
   /// Are we debugging sil serialization.
   bool DebugSerialization = false;
 
+  /// Optimization mode being used.
+  SILOptMode Optimization = SILOptMode::NotSet;
+
   enum AssertConfiguration: unsigned {
     // Used by standard library code to distinguish between a debug and release
     // build.

lib/Frontend/CompilerInvocation.cpp

@@ -881,18 +881,22 @@ static bool ParseSILArgs(SILOptions &Opts, ArgList &Args,
   if (const Arg *A = Args.getLastArg(OPT_O_Group)) {
     if (A->getOption().matches(OPT_Onone)) {
       IRGenOpts.Optimize = false;
+      Opts.Optimization = SILOptions::SILOptMode::None;
     } else if (A->getOption().matches(OPT_Ounchecked)) {
       // Turn on optimizations and remove all runtime checks.
       IRGenOpts.Optimize = true;
+      Opts.Optimization = SILOptions::SILOptMode::OptimizeUnchecked;
       // Removal of cond_fail (overflow on binary operations).
       Opts.RemoveRuntimeAsserts = true;
       Opts.AssertConfig = SILOptions::Fast;
     } else if (A->getOption().matches(OPT_Oplayground)) {
       // For now -Oplayground is equivalent to -Onone.
       IRGenOpts.Optimize = false;
+      Opts.Optimization = SILOptions::SILOptMode::None;
     } else {
       assert(A->getOption().matches(OPT_O));
       IRGenOpts.Optimize = true;
+      Opts.Optimization = SILOptions::SILOptMode::Optimize;
     }
   }