[RISCV] Pre-promote v1i1/v2i1/v4i1->i1/i2/i4 bitcasts before type legalization

Type legalization will convert the bitcast into a vector store and
scalar load.

Instead this patch widens the vector to v8i1 with undef, and bitcasts
it to i8. v8i1->i8 has custom handling for type legalization already to
bitcast to a v1i8 vector and use an extract_element.

The code here was lifted from X86's avx512 support.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D128099