[Comgr] Default to -O3 for all contexts

Previously Comgr required users/callers to manually supply
an optimization option. However, because -O3 is generally
accepted as useful, we can default to that if no option is
provided

Change-Id: I82b31f9260f0d8940944f794c661aee5f0480069

Author: lamb-j

amd/comgr/src/comgr-compiler.cpp

@@ -1035,6 +1035,9 @@ amd_comgr_status_t AMDGPUCompiler::addCompilationFlags() {
   // HIP headers depend on hsa.h which is in ROCM_DIR/include.
   ROCMIncludePath = (Twine(env::getROCMPath()) + "/include").str();
 
+  // Default to O3 for all contexts
+  Args.push_back("-O3");
+
   Args.push_back("-x");
 
   switch (ActionInfo->Language) {