Fix code section prefix for proper layout

[tejohnson]

Summary

r284533 added hot and cold section prefixes based on profile
information, to enable grouping of hot/cold functions at link time.
However, it used "cold" as the prefix for cold sections, but gold only
recognizes "unlikely" (which is used by gcc for cold sections).
Therefore, cold sections were not properly being grouped. Switch to
using "unlikely"

lib/CodeGen/CodeGenPrepare.cpp

bool CodeGenPrepare::runOnFunction(Function &F) {
   OptSize = F.optForSize();
 
   if (ProfileGuidedSectionPrefix) {
     ProfileSummaryInfo *PSI =
         getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI();
     if (PSI->isFunctionHotInCallGraph(&F))
       F.setSectionPrefix(".hot");
     else if (PSI->isFunctionColdInCallGraph(&F))
-      F.setSectionPrefix(".cold");
+      F.setSectionPrefix(".unlikely");
   }
 
   /// This optimization identifies DIV instructions that can be
   /// profitably bypassed and carried out with a shorter, faster divide.
   if (!OptSize && TLI && TLI->isSlowDivBypassed()) {
     const DenseMap<unsigned int, unsigned int> &BypassWidths =
        TLI->getBypassSlowDivWidths();
     BasicBlock* BB = &*F.begin();

test/Transforms/CodeGenPrepare/section.ll

 ; CHECK: cold_func{{.*}}!section_prefix ![[COLD_ID:[0-9]+]]
 ; The entry and the callsite are both cold
 define void @cold_func() !prof !16 {
   call void @hot_func(), !prof !18
   ret void
 }
 
 ; CHECK: ![[HOT_ID]] = !{!"function_section_prefix", !".hot"}
-; CHECK: ![[COLD_ID]] = !{!"function_section_prefix", !".cold"}
+; CHECK: ![[COLD_ID]] = !{!"function_section_prefix", !".unlikely"}
 !llvm.module.flags = !{!1}
 !1 = !{i32 1, !"ProfileSummary", !2}
 !2 = !{!3, !4, !5, !6, !7, !8, !9, !10}
 !3 = !{!"ProfileFormat", !"InstrProf"}
 !4 = !{!"TotalCount", i64 10000}
 !5 = !{!"MaxCount", i64 1000}
 !6 = !{!"MaxInternalCount", i64 1}
 !7 = !{!"MaxFunctionCount", i64 1000}