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

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).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191929 91177308-0d34-0410-b5e6-96231b3b80d8