If a conditional branch jumps to the same target, remove the condition

Given that large parts of inst combine is restricted to instructions which have one use, getting rid of a use on the condition can help the effectiveness of the optimizer. Also, it allows the condition to potentially be deleted by instcombine rather than waiting for another pass.

I noticed this completely by accident in another test case. It's not anything that actually came from a real workload.

p.s. We should probably do the same thing for switch instructions.

Differential Revision: http://reviews.llvm.org/D8220

llvm-svn: 231881