LoongArch: Fix instruction costs [PR112936]

Replace the instruction costs in loongarch_rtx_cost_data constructor
based on micro-benchmark results on LA464 and LA664.

This allows optimizations like "x * 17" to alsl, and "x * 68" to alsl
and slli.

gcc/ChangeLog:

	PR target/112936
	* config/loongarch/loongarch-def.cc
	(loongarch_rtx_cost_data::loongarch_rtx_cost_data): Update
	instruction costs per micro-benchmark results.
	(loongarch_rtx_cost_optimize_size): Set all instruction costs
	to (COSTS_N_INSNS (1) + 1).
	* config/loongarch/loongarch.cc (loongarch_rtx_costs): Remove
	special case for multiplication when optimizing for size.
	Adjust division cost when TARGET_64BIT && !TARGET_DIV32.
	Account the extra cost when TARGET_CHECK_ZERO_DIV and
	optimizing for speed.

gcc/testsuite/ChangeLog

	PR target/112936
	* gcc.target/loongarch/mul-const-reduction.c: New test.