[llvm-bugs] [Bug 141627] [RISCV] Unsigned atomics use signed load + zero-extension instead of unsigned loads

[LLVM Bugs via llvm-bugs]

Issue	141627
Summary	[RISCV] Unsigned atomics use signed load + zero-extension instead of unsigned loads
Labels	new issue
Assignees
Reporter	tom-rein

    The following code ([godbolt](https://godbolt.org/z/raqnhrGxa)):
```C++
#include <atomic>
unsigned char load(std::atomic_uchar &x) {
    return x.load(std::memory_order_relaxed);
}
```

Compiles into:
```
load(std::atomic<unsigned char>&):
        lb      a0, 0(a0)
 andi    a0, a0, 255
        ret
```

Instead of:
```
load(std::atomic<unsigned char>&):
        lbu     a0, 0(a0)
 ret
```
Same is the case for `unsigned short` and signed atomics casted to unsigned.
For `unsigned int` signed load is correct because of the ABI, but when it is zero-extended, llvm fails to combine the zero-extension with the load.
Non-atomic loads correctly result in unsigned loads.

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