[RISCV] Adjust RV64I data layout by using n32:64 in layout string

Although i32 type is illegal in the backend, RV64I has pretty good support for i32 types by using W instructions.

By adding n32 to the DataLayout string, middle end optimizations will consider i32 to be a native type. One known effect of this is enabling LoopStrengthReduce on loops with i32 induction variables. This can be beneficial because C/C++ code often has loops with i32 induction variables due to the use of `int` or `unsigned int`.

If this patch exposes performance issues, those are better addressed by tuning LSR or other passes.

Reviewed By: asb, frasercrmck

Differential Revision: https://reviews.llvm.org/D116735