Revert r274613 because it breaks the test suite with AVX512

This reverts most of r274613 (AKA r274626) 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.

(Recommit of r279782 which was broken due to a bad merge.)

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

llvm-svn: 279788