[APFloat] Set losesInfo on no-op convert

losesInfo would be left unset when no conversion needs to be done.  A
caller such as InstCombine's fitsInFPType would then branch on an
uninitialized value.

Caught using valgrind on an out-of-tree target.

Differential Revision: https://reviews.llvm.org/D46645

llvm-svn: 332087