SelectionDAGISel can now natively handle Switch instructions, in the same

SelectionDAGISel can now natively handle Switch instructions, in the same
manner that the LowerSwitch LLVM to LLVM pass does: emitting a binary
search tree of basic blocks.  The new approach has several advantages:
it is faster, it generates significantly smaller code in many cases, and
it paves the way for implementing dense switch tables as a jump table by
handling switches directly in the instruction selector.

This functionality is currently only enabled on x86, but should be safe for
every target.  In anticipation of making it the default, the cfg is now
properly updated in the x86, ppc, and sparc select lowering code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@27156 91177308-0d34-0410-b5e6-96231b3b80d8