Revert r274613 because it breaks the test suite with AVX512

This reverts most of r274613 and its follow-ups (r276347, r277289), due to
miscompiles in the test suite. The FastISel change was left in, because it
apparently fixes an unrelated issue.

This fixes 4 out of the 5 test failures in PR29112.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279782 91177308-0d34-0410-b5e6-96231b3b80d8
14 files changed