MIPS: Use SLL by 0 for 32-bit truncation in `__read_64bit_c0_split'

MIPS: Use SLL by 0 for 32-bit truncation in `__read_64bit_c0_split'

Optimize `__read_64bit_c0_split' and reduce the instruction count by 1,
observing that a DSLL/DSRA pair by 32, is equivalent to SLL by 0, which
architecturally truncates the value requested to 32 bits on 64-bit MIPS
hardware regardless of whether the input operand is or is not a properly
sign-extended 32-bit value.

Signed-off-by: Maciej W. Rozycki <macro@imgtec.com>
Reviewed-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/17399/
Signed-off-by: James Hogan <jhogan@kernel.org>