[PowerPC] Fold i1 extensions with other ops

Consider this function from our README.txt file:

  int foo(int a, int b) { return (a < b) << 4; }

We now explicitly track CR bits by default, so the comment in the README.txt
about not really having a SETCC is no longer accurate, but we did generate this
somewhat silly code:

        cmpw 0, 3, 4
        li 3, 0
        li 12, 1
        isel 3, 12, 3, 0
        sldi 3, 3, 4
        blr

which generates the zext as a select between 0 and 1, and then shifts the
result by a constant amount. Here we preprocess the DAG in order to fold the
results of operations on an extension of an i1 value into the SELECT_I[48]
pseudo instruction when the resulting constant can be materialized using one
instruction (just like the 0 and 1). This was not implemented as a DAGCombine
because the resulting code would have been anti-canonical and depends on
replacing chained user nodes, which does not fit well into the lowering
paradigm. Now we generate:

        cmpw 0, 3, 4
        li 3, 0
        li 12, 16
        isel 3, 12, 3, 0
        blr

which is less silly.

llvm-svn: 225203