BranchRelaxation: Expand unconditional branches first

It's likely if a conditional branch needs to be expanded, the following
unconditional branch will also need expansion. By expanding the
unconditional branch first, the conditional branch can be simply
inverted to jump over the inserted indirect branch block. If the
conditional branch is expanded first, it results in an additional
branch.

This avoids test regressions in future commits.

llvm-svn: 285722