[RISCV] Add a RISCV specific CodeGenPrepare pass.

Initial optimization is to convert (i64 (zext (i32 X))) to
(i64 (sext (i32 X))) if the dominating condition for the basic block
guaranteed the sign bit of X is zero.

This frequently occurs in loop preheaders where a signed induction
variable that can never be negative has been widened. There will be
a dominating check that the 32-bit trip count isn't negative or zero.
The check here is not restricted to that specific case though.

A i32->i64 sext is cheaper than zext on RV64 without the Zba
extension. Later optimizations can often remove the sext from the
preheader basic block because the dominating block also needs a sext to
evaluate the greater than 0 check.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D129732