SDISel should not optimize a unconditional branch following a conditional branch

SDISel should not optimize a unconditional branch following a conditional branch
when the unconditional branch destination is the fallthrough block. The
canonicalization makes it easier to allow optimizations on DAGs to invert
conditional branches. The branch folding pass (and AnalyzeBranch) will clean up
the unnecessary unconditional branches later.

This is one of the patches leading up to disabling codegen prepare critical edge
splitting.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@114630 91177308-0d34-0410-b5e6-96231b3b80d8