AArch64: Enable early scheduling for -O3 and higher (PR118351)

Enable the early scheduler on AArch64 for O3/Ofast.  This means GCC15 benefits
from much faster build times with -O2, but avoids the regressions in lbm which
is very sensitive to minor scheduling changes due to long FMA chains.

gcc:
	PR target/118351
	PR other/38768
	* common/config/aarch64/aarch64-common.cc: Enable early scheduling with
	-O3 and higher.
	* doc/invoke.texi (-fschedule-insns): Update comment.

gcc/common/config/aarch64/aarch64-common.cc

@@ -53,8 +53,10 @@ static const struct default_options aarch_option_optimization_table[] =
     { OPT_LEVELS_ALL, OPT_fomit_frame_pointer, NULL, 0 },
     /* Enable -fsched-pressure by default when optimizing.  */
     { OPT_LEVELS_1_PLUS, OPT_fsched_pressure, NULL, 1 },
-    /* Disable early scheduling due to high compile-time overheads.  */
+    /* Except for -O3 and higher, disable early scheduling due to high
+       compile-time overheads.  */
     { OPT_LEVELS_ALL, OPT_fschedule_insns, NULL, 0 },
+    { OPT_LEVELS_3_PLUS, OPT_fschedule_insns, NULL, 1 },
     /* Enable redundant extension instructions removal at -O2 and higher.  */
     { OPT_LEVELS_2_PLUS, OPT_free, NULL, 1 },
     { OPT_LEVELS_2_PLUS, OPT_mearly_ra_, NULL, AARCH64_EARLY_RA_ALL },

gcc/doc/invoke.texi

@@ -13507,7 +13507,9 @@ helps machines that have slow floating point or memory load instructions
 by allowing other instructions to be issued until the result of the load
 or floating-point instruction is required.
 
-Enabled at levels @option{-O2}, @option{-O3}.
+Conventionally enabled at optimization levels @option{-O2} and @option{-O3}.
+However, many targets override this behavior.  For example, on x86, it is
+disabled at all levels, while on AArch64, it is enabled only at @option{-O3}.
 
 @opindex fschedule-insns2
 @item -fschedule-insns2