Improve addressing of TI/TFmode

Improve addressing of TI/TFmode

In https://gcc.gnu.org/ml/gcc-patches/2017-06/msg01125.html Jiong
pointed out some addressing inefficiencies due to a recent change in
regcprop (https://gcc.gnu.org/ml/gcc-patches/2017-04/msg00775.html).

This patch improves aarch64_legitimize_address_displacement to split
unaligned offsets of TImode and TFmode accesses.  The resulting code
is better and no longer relies on the original regcprop optimization.

For the test we now produce:

	add	x1, sp, 4
	stp	xzr, xzr, [x1, 24]

rather than:

        mov     x1, sp
        add     x1, x1, 28
        stp     xzr, xzr, [x1]

    gcc/
	* config/aarch64/aarch64.c (aarch64_legitimize_address_displacement):
	Improve unaligned TImode/TFmode base/offset split.

    testsuite/
	* gcc.target/aarch64/ldp_stp_unaligned_2.c: New file.

From-SVN: r254111