LoongArch: Add alslsi3_extend

Following the instruction cost fix, we are generating

    alsl.w $a0, $a0, $a0, 4

instead of

    li.w  $t0, 17
    mul.w $a0, $t0

for "x * 4", because alsl.w is 4 times faster than mul.w.  But we didn't
have a sign-extending pattern for alsl.w, causing an extra slli.w
instruction generated to sign-extend $a0.  Add the pattern to remove the
redundant extension.

gcc/ChangeLog:

	* config/loongarch/loongarch.md (alslsi3_extend): New
	define_insn.