[RISCV] Custom-legalise i32 SDIV/UDIV/UREM on RV64M

Follow the same custom legalisation strategy as used in D57085 for
variable-length shifts (see that patch summary for more discussion). Although
we may lose out on some late-stage DAG combines, I think this custom
legalisation strategy is ultimately easier to reason about.

There are some codegen changes in rv64m-exhaustive-w-insts.ll but they are all
neutral in terms of the number of instructions.

Differential Revision: https://reviews.llvm.org/D57096

llvm-svn: 352171