ARM: correctly update CFG when splitting BB to fix branch.

Because the block-splitting code is multi-purpose, we have to meddle with the
branches when using it to fixup a conditional branch destination. We got the
code right, but forgot to update the CFG so the verifier complained when
expensive checks were on.

Probably harmless since constant-islands comes so late, but best to fix it
anyway.

llvm-svn: 318148