BranchRelaxation: Fix handling of blocks with multiple conditional

branches

Looping over all terminators exposed AArch64 tests hitting
an assert from analyzeBranch failing. I believe these cases
were miscompiled before.

e.g.
  fcmp s0, s1
  b.ne LBB0_1
  b.vc LBB0_2
  b LBB0_2
LBB0_1:
  ; Large block
LBB0_2:
 ; ...

Both of the individual conditional branches need to
be expanded, since neither can reach the final block.

Split the original block into ones which analyzeBranch
will be able to understand.

llvm-svn: 279499