SimplifyCFG: preserve branch-weight metadata when creating a new switch from

SimplifyCFG: preserve branch-weight metadata when creating a new switch from
a pair of switch/branch where both depend on the value of the same variable and
the default case of the first switch/branch goes to the second switch/branch.

Code clean up and fixed a few issues:
1> handling the case where some cases of the 2nd switch are invalidated
2> correctly calculate the weight for the 2nd switch when it is a conditional eq

Testing case is modified from Alastair's original patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@163635 91177308-0d34-0410-b5e6-96231b3b80d8