[JumpThreading] Fix incorrect fold conditional after indirectbr/callbr

Fixes bug 40992: https://bugs.llvm.org/show_bug.cgi?id=40992

There is potential for miscompiled code emitted from JumpThreading when
analyzing a block with one or more indirectbr or callbr predecessors. The
ProcessThreadableEdges() function incorrectly folds conditional branches
into an unconditional branch.

This patch prevents incorrect branch folding without fully pessimizing
other potential threading opportunities through the same basic block.

This IR shape was manually fed in via opt and is unclear if clang and the
full pass pipeline will ever emit similar code shapes.

Thanks to Matthias Liedtke for the bug report and simplified IR example.

Differential Revision: https://reviews.llvm.org/D60284

llvm-svn: 357930