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>

arch/mips/include/asm/mipsregs.h

@@ -1355,19 +1355,17 @@ do {									\
 	if (sel == 0)							\
 		__asm__ __volatile__(					\
 			".set\tmips64\n\t"				\
-			"dmfc0\t%M0, " #source "\n\t"			\
-			"dsll\t%L0, %M0, 32\n\t"			\
-			"dsra\t%M0, %M0, 32\n\t"			\
-			"dsra\t%L0, %L0, 32\n\t"			\
+			"dmfc0\t%L0, " #source "\n\t"			\
+			"dsra\t%M0, %L0, 32\n\t"			\
+			"sll\t%L0, %L0, 0\n\t"				\
 			".set\tmips0"					\
 			: "=r" (__val));				\
 	else								\
 		__asm__ __volatile__(					\
 			".set\tmips64\n\t"				\
-			"dmfc0\t%M0, " #source ", " #sel "\n\t"		\
-			"dsll\t%L0, %M0, 32\n\t"			\
-			"dsra\t%M0, %M0, 32\n\t"			\
-			"dsra\t%L0, %L0, 32\n\t"			\
+			"dmfc0\t%L0, " #source ", " #sel "\n\t"		\
+			"dsra\t%M0, %L0, 32\n\t"			\
+			"sll\t%L0, %L0, 0\n\t"				\
 			".set\tmips0"					\
 			: "=r" (__val));				\
 	local_irq_restore(__flags);					\