Eliminate the first have of the optimization which eliminates BRCOND

Eliminate the first have of the optimization which eliminates BRCOND
when the condition is constant. This optimization shouldn't be
necessary, because codegen shouldn't be able to find dead control
paths that the IR-level optimizer can't find. And it's undesirable,
because it encourages bugpoint to leave "br i1 false" branches
in its output. And it wasn't updating the CFG.

I updated all the tests I could, but some tests are too reduced
and I wasn't able to meaningfully preserve them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@106748 91177308-0d34-0410-b5e6-96231b3b80d8