[RISCV] Use std::make_tuple to make some toolchains happy again

My toolchain (LLVM 8.0, libstdc++ 5.4.0) complained with:

12:38:19 ../lib/Target/RISCV/RISCVISelLowering.cpp:1717:12: error: chosen constructor is explicit in copy-initialization
12:38:19     return {RISCVISD::VECREDUCE_FADD, Op.getOperand(0),
12:38:19            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
12:38:19 /proj/flexasic/app/llvm/8.0/bin/../lib/gcc/x86_64-unknown-linux-gnu/5.4.0/../../../../include/c++/5.4.0/tuple:479:19: note: explicit constructor declared here
12:38:19         constexpr tuple(_UElements&&... __elements)
12:38:19                   ^
12:38:19 ../lib/Target/RISCV/RISCVISelLowering.cpp:1720:12: error: chosen constructor is explicit in copy-initialization
12:38:19     return {RISCVISD::VECREDUCE_SEQ_FADD, Op.getOperand(1), Op.getOperand(0)};
12:38:19            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
12:38:19 /proj/flexasic/app/llvm/8.0/bin/../lib/gcc/x86_64-unknown-linux-gnu/5.4.0/../../../../include/c++/5.4.0/tuple:479:19: note: explicit constructor declared here
12:38:19         constexpr tuple(_UElements&&... __elements)
12:38:19                   ^
12:38:19 2 errors generated.

This commit adds explicit calls to std::make_tuple to work around
the problem.