R600/SI: Use ZeroOrNegativeOneBooleanContent

This sort of doesn't matter since the setcc type is i1, but
this previously was using the default UndefinedBooleanContent. This
makes it more consistent with R600. This enables more optimizations
which typically give up on UndefinedBooleanContent. For example,
there is already a special case target DAG combine for
setcc + sext which can be eliminated in favor of what the generic
DAG combiner can do if it assumes boolean values are sign extended.
Since -1 is an inline immediate, using it is basically free and the
backend already uses it when a boolean value is needed in a wider type.