Recommit "[GlobalISel] Add IRTranslator support for G_FFLOOR"

After the changes introduced in r353586, this instruction doesn't cause any
issues for any backend.

Original review: https://reviews.llvm.org/D57485

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353720 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed