[RISCV][test] Add tests for RISCVInstrInfo::describeLoadedValue (#76041)

Tests are in preparation for adding handling of the load of a constant
value as Mips does (noted in

<https://github.com/llvm/llvm-project/pull/72356#discussion_r1395203532>).

I've opted to implement these tests as a C++ unit test as on balance I
_think_ it's easier to follow and maintain than .mir tests trying to
indirectly test this function. That said, you see the limitations with
the test of describeLoadedValue on a memory operation where we'd rather
pass `MachinePointerInfo::getFixedStack` but can't because we'd need to
then ensure the necessary stack metadata for the function is present.