AArch64 NCG: `signExtendReg` W32 emits SXTH instead of SXTW
## Summary
`signExtendReg` in `compiler/GHC/CmmToAsm/AArch64/CodeGen.hs` (~line 1450) uses `SXTH` (sign-extend halfword, 16-bit) for the `W32 | otherwise` case. It should use `SXTW` (sign-extend word, 32-bit) to correctly sign-extend a 32-bit value to 64 bits.
`SXTW` is commented out in `compiler/GHC/CmmToAsm/AArch64/Instr.hs` (line 610).
## Environment
* GHC version used: 9.15
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