[SVE] Change some bfloat lane intrinsics to use i32 immediates

Almost all of the other SVE LLVM IR intrinsics take i32 values
for lane indices or other immediates. We should bring the bfloat
intrinsics in line with that. It will also make it easier to
add support for the SVE2.1 float intrinsics in future, since
they reuse the same underlying instruction classes.

I've maintained backwards compatibility with the old i64 variants
and used the autoupgrade mechanism.

Differential Revision: https://reviews.llvm.org/D138788