AArch64 NCG: MOVK `regUsageOfInstr` missing read of destination
## Summary
In `compiler/GHC/CmmToAsm/AArch64/Instr.hs` (line 117), `regUsageOfInstr` for `MOVK` is:
```haskell
MOVK dst src -> usage (regOp src, regOp dst)
```
`MOVK` is a read-modify-write instruction: it reads the destination register, replaces one 16-bit lane, and writes the result back. The current definition only lists `dst` in the write set, not the read set. This means the register allocator may reuse the destination register prematurely, clobbering a live value.
## Proposed fix
```haskell
MOVK dst src -> usage (regOp src ++ regOp dst, regOp dst)
```
Add `dst` to the read set so the allocator knows the old value is still needed.
## Environment
* GHC version used: 9.15
issue