[RISCV] Use stack temporary to splat two GPRs into SEW=64 vector on RV32.

Rather than doing splatting each separately and doing bit manipulation
to merge them in the vector domain, copy the data to the stack
and splat it using a strided load with x0 stride. At least on
some implementations this vector load is optimized to not do
a load for each element.

This is equivalent to how we move i64 to f64 on RV32.

I've only implemented this for the intrinsic fallbacks in this
patch. I think we do similar splatting/shifting/oring in other
places. If this is approved, I'll refactor the others to share
the code.

Differential Revision: https://reviews.llvm.org/D101002