Re-commit r235560: Switch lowering: extract jump tables and bit tests before building...

Re-commit r235560: Switch lowering: extract jump tables and bit tests before building binary tree (PR22262)

Third time's the charm. The previous commit was reverted as a
reverse for-loop in SelectionDAGBuilder::lowerWorkItem did 'I--'
on an iterator at the beginning of a vector, causing asserts
when using debugging iterators. This commit fixes that.

llvm-svn: 235608