Improve aarch64_legitimize_address - avoid splitting the offset if it is

Improve aarch64_legitimize_address - avoid splitting the offset if it is
supported.  When we do split, take the mode size into account.  BLKmode
falls into the unaligned case but should be treated like LDP/STP.
This improves codesize slightly due to fewer base address calculations:

int f(int *p) { return p[5000] + p[7000]; }

Now generates:

f:
	add	x0, x0, 16384
	ldr	w1, [x0, 3616]
	ldr	w0, [x0, 11616]
	add	w0, w1, w0
	ret

instead of:

f:
	add	x1, x0, 16384
	add	x0, x0, 24576
	ldr	w1, [x1, 3616]
	ldr	w0, [x0, 3424]
	add	w0, w1, w0
	ret

    gcc/
	* config/aarch64/aarch64.c (aarch64_legitimize_address):
	Avoid use of base_offset if offset already in range.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@240026 138bc75d-0d04-0410-961f-82ee72b054a4