[RISCV] Handle redundant SplitF64+BuildPairF64 pairs in a DAGCombine

r343712 performed this optimisation during instruction selection. As Eli 
Friedman pointed out in post-commit review, implementing this as a DAGCombine 
might allow opportunities for further optimisations.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@343741 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed