Merge 00f3579aea6e3d4a4b7464c3db47294f71cef9e4 to 11.0

[topperc]

Bugzilla Link	46740
Resolution	FIXED
Resolved on	Jul 16, 2020 07:58
Version	trunk
OS	All
Blocks	#46070
CC	@zmodem

Extended Description

This reverts most of a 5 patch series due to reports of miscompiles.

The patches are
1cf6f21 [IR] Disable select ? C : undef -> C fold in ConstantFoldSelectInstruction unless we know C isn't poison.
469da66 [InstSimplify] Re-enable select ?, undef, X -> X transform when X is provably not poison
122b064 [InstSimplify] Don't fold vectors of partial undef in SimplifySelectInst if the non-undef element value might produce poison
ac0af12 [InstSimplify] Add test cases for opportunities to fold select ?, X, undef -> X when we can prove X isn't poison
9b1e953 [InstSimplify] Remove select ?, undef, X -> X and select ?, X, undef -> X transforms

Some of them added new test cases which I left but change the CHECK lines.

[topperc]

Miscompile discussion is here https://reviews.llvm.org/D83360

[zmodem]

Pushed in 529f2e0. Thanks!