[RISCV] Add test cases to show an issue with our fcvt.wu isel patterns on RV64.

The pattern we match is (sext_inreg (assertzexti32 (fp_to_uint)), i32). If
the assertzexti32 has an additional user we'll end up emitting
an fcvt.wu and an fcvt.lu.

This can happen if the original fp_to_uint before type legalization
has one user that causes a sext_inreg to be emitted and one that
doesn't.