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.

llvm-svn: 279782