[RISCV] Improve 64-bit integer constant materialization for more cases.

[RISCV] Improve 64-bit integer constant materialization for more cases.

For positive constants we try shifting left to remove leading zeros
and fill the bottom bits with 1s. We then materialize that constant
shift it right.

This patch adds a new strategy to try filling the bottom bits with
zeros instead. This catches some additional cases.