[CodegenPrepare] Guard against degenerate branches

Summary:
Guard against a potential crash observed in https://github.com/JuliaLang/julia/issues/32994#issuecomment-524249628
If two branches are collapsed we can encounter a degenerate conditional branch `TBB==FBB`.
The subsequent code assumes that they differ, so we exit out early.

Reviewers: ributzka, spatel

Subscribers: loladiro, dexonsmith, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66657