Merging r371088 and r371095:

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r371088 | spatel | 2019-09-05 18:58:18 +0200 (Thu, 05 Sep 2019) | 1 line

[x86] add test for horizontal math bug (PR43225); NFC
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r371095 | spatel | 2019-09-05 19:28:17 +0200 (Thu, 05 Sep 2019) | 3 lines

[x86] fix horizontal math bug exposed by improved demanded elements analysis (PR43225)

https://bugs.llvm.org/show_bug.cgi?id=43225
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llvm-svn: 371178