Pull fptrunc's upwards through selects when one of the select's selectands was...

Pull fptrunc's upwards through selects when one of the select's selectands was a constant.  This has a number of benefits, including producing small immediates (easier to materialize, smaller constant pools) as well as being more likely to allow the fptrunc to fuse with a preceding instruction (truncating selects are unusual).

llvm-svn: 191929