PGO branch weight: update edge weights in IfConverter.

This commit only handles IfConvertTriangle. To update edge weights
of a successor, one interface is added to MachineBasicBlock:
/// Set successor weight of a given iterator.
setSuccWeight(succ_iterator I, uint32_t weight)

An existing testing case test/CodeGen/Thumb2/v8_IT_5.ll is updated,
since we now correctly update the edge weights, the cold block
is placed at the end of the function and we jump to the cold block.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200428 91177308-0d34-0410-b5e6-96231b3b80d8