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.
<rdar://problem/15936507>

PyTuple_SetItem steals a reference to the item it inserts in the tuple
This, plus the Py_XDECREF of the tuple a few lines below, causes our session dictionary to go away after the first time a SWIG layer function is called - with disastrous effects for the first subsequent attempt to use any functionality in ScriptInterpreterPython
This fixes it