[DAGCombiner] remove hasOneUse() check from fadd constants transform

We're constant folding here, so we shouldn't check uses. This matches
the IR optimizer behavior.

The x86 test shows the expected win. The AArch64 test shows something
else. This only seems to happen if the "generic" AArch64 CPU model is 
used by MachineCombiner, so I'll file a bug report to follow-up.

llvm-svn: 334608