[CodeGen] Mark the promoted FCOPYSIGN result FP_ROUND as TRUNCating.

[CodeGen] Mark the promoted FCOPYSIGN result FP_ROUND as TRUNCating.

Now that we can properly promote mismatched FCOPYSIGNs (r244858), we
can mark the FP_ROUND on the result as truncating, to expose folding.

FCOPYSIGN doesn't change anything but the sign bit, so
  (fp_round (fcopysign (fpext a), b))
is equivalent to (modulo the sign bit):
  (fp_round (fpext a))
which is a no-op.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@244862 91177308-0d34-0410-b5e6-96231b3b80d8