[RISCV] Lower experimental_get_vector_length intrinsic to vsetvli for some cases.

This patch lowers to vsetvli when the AVL is i32 or XLenVT and
the VF is a power of 2 in the range [1, 64]. VLEN=32 is not supported
as we don't have a valid type mapping for that. VF=1 is not supported
with Zve32* only.

The element width is used to set the SEW for the vsetvli if possible.
Otherwise we use SEW=8.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D150824