[HotColdSplitting] Attach MinSize to outlined code

[vsk]

Summary

Outlined code is cold by assumption, so it makes more sense to optimize
it for minimal code size versus performance.

After r344869 moved the splitting pass to the end of the IR pipeline,
this does not result in much of a code size reduction. Only a small
number backend transforms make use of the MinSize hint.

Running LNT on x86_64, I see that 33/1020 binaries shrink for a total of
919 bytes of TEXT reduction. I didn't measure a significant performance
impact.

[sebpop]

ok

llvm/trunk/lib/Transforms/IPO/HotColdSplitting.cpp

if (Function *OutF = CE.extractCodeRegion()) {
     CallInst *CI = cast<CallInst>(U);
     CallSite CS(CI);
     NumColdSESEOutlined++;
     if (GetTTI(*OutF).useColdCCForColdCall(*OutF)) {
       OutF->setCallingConv(CallingConv::Cold);
       CS.setCallingConv(CallingConv::Cold);
     }
     CI->setIsNoInline();
+
+    // Try to make the outlined code as small as possible on the assumption
+    // that it's cold.
+    assert(!OutF->hasFnAttribute(Attribute::OptimizeNone) &&
+           "An outlined function should never be marked optnone");
+    OutF->addFnAttr(Attribute::MinSize);
+
     LLVM_DEBUG(llvm::dbgs() << "Outlined Region: " << *OutF);
     ORE.emit([&]() {
       return OptimizationRemark(DEBUG_TYPE, "HotColdSplit",
                                 &*Region[0]->begin())
              << ore::NV("Original", OrigF) << " split cold code into "
              << ore::NV("Split", OutF);
     });
     return OutF;

llvm/trunk/test/Transforms/HotColdSplit/minsize.ll

+; RUN: opt -hotcoldsplit -S < %s | FileCheck %s
+
+; CHECK-LABEL: @fun
+; CHECK: codeRepl:
+; CHECK-NEXT: call void @fun_if.else
+
+define void @fun() {
+entry:
+  br i1 undef, label %if.then, label %if.else
+
+if.then:
+  ret void
+
+if.else:
+  br label %if.then4
+
+if.then4:
+  br i1 undef, label %if.then5, label %if.end
+
+if.then5:
+  br label %cleanup
+
+if.end:
+  br label %cleanup
+
+cleanup:
+  %cleanup.dest.slot.0 = phi i32 [ 1, %if.then5 ], [ 0, %if.end ]
+  unreachable
+}
+
+; CHECK: define {{.*}} @fun_if.else{{.*}}#[[outlined_func_attr:[0-9]+]]
+; CHECK: attributes #[[outlined_func_attr]] = { {{.*}}minsize