[RISCV] Pre-commit tests for D101342

These tests show inefficient sign extension for AMOs on RISC-V. The
normal CodeGen tests use anyext return values, but if marked signext
then we end up generating unnecessary sign extension instructions. This
can be seen when compiling C that returns an i32 (signed or unsigned),
where the calling convention results in a signext return value.

Author: jrtc27