Fix a dagga combiner bug: avoid creating illegal constant.

Is this really a winning transformation?
fold (shl (srl x, c1), c2) -> (shl (and x, (shl -1, c1)), (sub c2, c1)) or                                                                              
                              (srl (and x, (shl -1, c1)), (sub c1, c2))

llvm-svn: 76535