Reduce cost of MEM (A + imm).

Reduce cost of MEM (A + imm).

For MEM, rtx_cost iterates each subrtx, and adds up the costs,
so for MEM (reg) and MEM (reg + 4), the former costs 5,
the latter costs 9, it is not accurate for x86. Ideally
address_cost should be used, but it reduce cost too much.
So current solution is make constant disp as cheap as possible.

gcc/ChangeLog:

	PR target/67325
	* config/i386/i386.cc (ix86_rtx_costs): Reduce cost of MEM (A
	+ imm) to "cost of MEM (A)" + 1.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr67325.c: New test.