Fixed an inappropriate use of BuildPairF64 when compiling for MIPS32 with FP64
which resulted in an impossible constraint on the register allocation. It now
uses BuildPairF64_64.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195007 91177308-0d34-0410-b5e6-96231b3b80d8