Decrease cortexa57_extra_costs's alu.shift_reg

Like I mentioned in https://gcc.gnu.org/ml/gcc/2020-01/msg00157.html,
The shift by a register should be just COSTS_N_INSNS (1) rather than
COSTS_N_INSNS (2).  This allows lshift_cheap_p to return true now
and converting switches to be using shift and other like
structures.  I noticed this difference when I was working
through PR 93131 and understanding what reassoc could handle.

ChangeLog:
* config/arm/aarch-cost-tables.h (cortexa57_extra_costs): Change
alu.shift_reg to 0.

gcc/ChangeLog

+2020-01-24  Andrew Pinski  <apinski@marvell.com>
+
+	* config/arm/aarch-cost-tables.h (cortexa57_extra_costs): Change
+	alu.shift_reg to 0.
+
 2020-01-24  Jeff Law  <law@redhat.com>
 
 	PR target/13721

gcc/config/arm/aarch-cost-tables.h

@@ -235,7 +235,7 @@ const struct cpu_cost_table cortexa57_extra_costs =
     0,                 /* arith.  */
     0,                 /* logical.  */
     0,                 /* shift.  */
-    COSTS_N_INSNS (1), /* shift_reg.  */
+    0,			/* shift_reg.  */
     COSTS_N_INSNS (1), /* arith_shift.  */
     COSTS_N_INSNS (1), /* arith_shift_reg.  */
     COSTS_N_INSNS (1), /* log_shift.  */