[DAGCombiner] Don't emit select_cc from visitSINT_TO_FP/visitUINT_TO_FP. Use plain select instead.

Select_cc isn't used by all targets. X86 doesn't have optimizations
for it.

Since we already know the input to the sint_to_fp/uint_to_fp is
a setcc we can just emit a plain select using that setcc as the
condition. Other DAG combines can turn that into a select_cc on
targets that support it.

Differential Revision: https://reviews.llvm.org/D75415