[RISCV] Lower reverse shuffles of fixed i1 vectors to vbrev.v

If we can fit an entire vector of i1 into a single element, e.g. v32i1 ->
v1i32, then we can reverse it via vbrev.v.
We need to handle the case where the vector doesn't exactly fit into the larger
element type, e.g. v4i1 -> v1i8. In this case we shift up the reversed bits
afterwards.

Reviewed By: fakepaper56, 4vtomat

Differential Revision: https://reviews.llvm.org/D157614