[IfConversion] Only renormalize probabilities if branches are analyzable

If a block has non-analyzable branches, the listed successors don't need
to add up to one. For example, if a block has a conditional tail call,
that tail call will not have a corresponding successor in the successor
list, but will still be a possible branch.

Differential Revision: https://reviews.llvm.org/D30556

llvm-svn: 297054