Merging r351577:

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r351577 | ssijaric | 2019-01-18 11:34:20 -0800 (Fri, 18 Jan 2019) | 5 lines

Fix the buildbot issue introduced by r351421

The EXPENSIVE_CHECK x86_64 Windows buildbot is failing due to this change. Fix
the map access.

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llvm-svn: 363006

Author: tstellar

llvm/lib/MC/MCWin64EH.cpp

@@ -522,7 +522,7 @@ static void ARM64EmitUnwindInfo(MCStreamer &streamer, WinEH::FrameInfo *info) {
     if (MatchingEpilog) {
       assert(EpilogInfo.find(MatchingEpilog) != EpilogInfo.end() &&
              "Duplicate epilog not found");
-      EpilogInfo[EpilogStart] = EpilogInfo[MatchingEpilog];
+      EpilogInfo[EpilogStart] = EpilogInfo.lookup(MatchingEpilog);
       // Clear the unwind codes in the EpilogMap, so that they don't get output
       // in the logic below.
       EpilogInstrs.clear();