teach selection dag mask tracking about the fact that select_cc operates like

select.  Also teach it that the bit count instructions can only set the low bits
of the result, depending on the size of the input.

This allows us to compile this:

int %eq0(int %a) {
        %tmp.1 = seteq int %a, 0                ; <bool> [#uses=1]
        %tmp.2 = cast bool %tmp.1 to int                ; <int> [#uses=1]
        ret int %tmp.2
}

To this:

_eq0:
        cntlzw r2, r3
        srwi r3, r2, 5
        blr

instead of this:

_eq0:
        cntlzw r2, r3
        rlwinm r3, r2, 27, 31, 31
        blr

when setcc is marked illegal on ppc (which restores parity to non-illegal
setcc).  Thanks to Nate for pointing this out.

llvm-svn: 23013