SimplifyCFG: Make the switch-to-lookup table transformation store the

SimplifyCFG: Make the switch-to-lookup table transformation store the
tables in bitmaps when they fit in a target-legal register.

This saves some space, and it also allows for building tables that would
otherwise be deemed too sparse.

One interesting case that this hits is example 7 from
http://blog.regehr.org/archives/320. We currently generate good code
for this when lowering the switch to the selection DAG: we build a
bitmask to decide whether to jump to one block or the other. My patch
will result in the same bitmask, but it removes the need for the jump,
as the return value can just be retrieved from the mask.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164684 91177308-0d34-0410-b5e6-96231b3b80d8