Decrease cortexa57_extra_costs's alu.shift_reg

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.