expansion: make layout of x_shift*cost[][][] more efficient

when debugging expmed.[ch] for PR/108987 saw that some of the cost arrays have
less than ideal layout as follows:

   x_shift*cost[0..63][speed][modes]

We would want speed to be first index since a typical compile will have
that fixed, followed by mode and then the shift values.

It should be non-functional from compiler semantics pov, except
executing slightly faster due to better locality of shift values for
given speed and mode. And also a bit more intutive when debugging.

gcc/Changelog:

	* expmed.h (x_shift*_cost): convert to int [speed][mode][shift].
	(shift*_cost_ptr ()): Access x_shift*_cost array directly.

Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>