LoongArch: Adjust widen-mul rtx cost

LoongArch: Adjust widen-mul rtx cost

Equalize the rtx cost of mulw.d.w[u] with mul.d.

There are two main benefits:
1. Reducing the number of instructions.
    -       lu32i.d	$r12,0
    -       bstrpick.d	$r4,$r4,31,0
    -       mul.d	$r4,$r4,$r12
    -       srli.d	$r4,$r4,33
    ---
    +       mulh.wu	$r4,$r4,$r12
    +       bstrpick.d	$r4,$r4,31,1

2. Help with the replacement of the high-latency div.w.
    -       addi.w	$r12,$r0,3
    -       div.w	$r4,$r4,$r12
    ---
    +       lu12i.w	$r13,349525
    +       ori		$r13,$r13,1366
    +       mulw.d.w	$r12,$r4,$r13
    +       srai.w 	$r4,$r4,31
    +       srli.d	$r12,$r12,32
    +       sub.w	$r4,$r12,$r4

gcc/ChangeLog:

	* config/loongarch/loongarch.cc (loongarch_rtx_costs):
	Ignore the cost impact of SIGN_EXTEND/ZERO_EXTEND.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/widen-mul-rtx-cost-signed.c: New test.
	* gcc.target/loongarch/widen-mul-rtx-cost-unsigned.c: New test.