type constraint. This lets tblgen realize that it doesn't need any dynamic
type checks for fextend/fround on PPC (and many other targets), because there
are only two fp types.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23730 91177308-0d34-0410-b5e6-96231b3b80d8