Reland "[NFCI][SimplifyCFG] simplifyCondBranch(): assert that branch is non-tautological""

The commit originally unearthed a problem, reported as
https://reviews.llvm.org/rGf30a7dff8a5b32919951dcbf92e4a9d56c4679ff#1019890
Now that the problem has been fixed, and the assertion no longer fires,
let's see if there are other cases it fires on.

This reverts commit 5c8c24d2,
relanding commit f30a7dff.