Prevent the creation of empty (forwarding) blocks resulting from nested ifs.

Summary:
Nested if statements can generate empty BBs whose terminator branches 
unconditionally to its successor. These branches are not eliminated
to help generate better line number information in some cases, but there
is no reason to keep the empty blocks that result from nested ifs.

Reviewers: mehdi_amini, dblaikie, echristo

Subscribers: mehdi_amini, cfe-commits

Differential review: http://reviews.llvm.org/D11360
 

llvm-svn: 275115