LoongArch: Modify the address calculation logic for obtaining array element values...

LoongArch: Modify the address calculation logic for obtaining array element values through fp.

Modify address calculation logic from (((a x C) + fp) + offset) to ((fp + offset) + a x C).
Thereby modifying the register dependencies and optimizing the code.
The value of C is 2 4 or 8.

The following is the assembly code before and after a loop modification in spec2006 401.bzip:

                 old                      |                 new
 735 .L71:                                |  735 .L71:
 736         slli.d  $r12,$r15,2          |  736         slli.d  $r12,$r15,2
 737         ldx.w   $r13,$r22,$r12       |  737         ldx.w   $r13,$r22,$r12
 738         addi.d  $r15,$r15,-1         |  738         addi.d  $r15,$r15,-1
 739         slli.w  $r16,$r15,0          |  739         slli.w  $r16,$r15,0
 740         addi.w  $r13,$r13,-1         |  740         addi.w  $r13,$r13,-1
 741         slti    $r14,$r13,0          |  741         slti    $r14,$r13,0
 742         add.w   $r12,$r26,$r13       |  742         add.w   $r12,$r26,$r13
 743         maskeqz $r12,$r12,$r14       |  743         maskeqz $r12,$r12,$r14
 744         masknez $r14,$r13,$r14       |  744         masknez $r14,$r13,$r14
 745         or      $r12,$r12,$r14       |  745         or      $r12,$r12,$r14
 746         ldx.bu  $r14,$r30,$r12       |  746         ldx.bu  $r14,$r30,$r12
 747         lu12i.w $r13,4096>>12        |  747         alsl.d  $r14,$r14,$r18,2
 748         ori     $r13,$r13,432        |  748         ldptr.w $r13,$r14,0
 749         add.d   $r13,$r13,$r3        |  749         addi.w  $r17,$r13,-1
 750         alsl.d  $r14,$r14,$r13,2     |  750         stptr.w $r17,$r14,0
 751         ldptr.w $r13,$r14,-1968      |  751         slli.d  $r13,$r13,2
 752         addi.w  $r17,$r13,-1         |  752         stx.w   $r12,$r22,$r13
 753         st.w    $r17,$r14,-1968      |  753         ldptr.w $r12,$r19,0
 754         slli.d  $r13,$r13,2          |  754         blt     $r12,$r16,.L71
 755         stx.w   $r12,$r22,$r13       |  755         .align  4
 756         ldptr.w $r12,$r18,-2048      |  756
 757         blt     $r12,$r16,.L71       |  757
 758         .align  4                    |  758

This patch is ported from riscv's commit r14-3111.

gcc/ChangeLog:

	* config/loongarch/loongarch.cc (mem_shadd_or_shadd_rtx_p): New function.
	(loongarch_legitimize_address): Add logical transformation code.