[AArch64] Improve TARGET_LEGITIMIZE_ADDRESS_P hook

[AArch64] Improve TARGET_LEGITIMIZE_ADDRESS_P hook

currently, aarch64 LEGITIMIZE_ADDRESS_P hook will reject all "reg + offset"
address given "offset" is beyond supported range.

while this may be too strict. we should honor the "strict_p" parameter in the
hook. before reload, we accept all offset if it's a frame access, because the
offset may change during later register elimination.

the early reject of "reg + offset" may cause extra registers created, and if
that register live range is across function invoking then callee saved reg
needed, thus introduce extra reg save/restore also.

give a simple example as:

int
test15 (void)
{
   unsigned char a[480];
   initialize_array (a, 480);

   if (a[0] == 0x10)
     return 1;

   return 0;
}

.S before the patch
(-O2 -fPIC)
===
test15:
         sub     sp, sp, #480
         mov     w1, 480
         stp     x29, x30, [sp, -32]!
         add     x29, sp, 0
         str     x19, [sp, 16]
         add     x19, x29, 32
         mov     x0, x19
         bl      initialize_array
         ldrb    w0, [x19]
         ldr     x19, [sp, 16]
         ldp     x29, x30, [sp], 32
         cmp     w0, 16
         cset    w0, eq
         add     sp, sp, 480
         ret

.S after the patch
===
test15:
         stp     x29, x30, [sp, -496]!
         mov     w1, 480
         add     x29, sp, 0
         add     x0, x29, 16
         bl      initialize_array
         ldrb    w0, [x29, 16]
         ldp     x29, x30, [sp], 496
         cmp     w0, 16
         cset    w0, eq
         ret

gcc/
  * config/aarch64/aarch64.c (aarch64_classify_address): Accept all offset for
  frame access when strict_p is false.

gcc/testsuite
   * gcc.target/aarch64/legitimize_stack_var_before_reload_1.c: New testcase.

From-SVN: r213488