[AArch64] Optimise aarch64_add_offset for SVE VL constants

[AArch64] Optimise aarch64_add_offset for SVE VL constants

aarch64_add_offset contains code to decompose all SVE VL-based constants
into native operations.  The worst-case fallback is to load the number
of SVE elements into a register and use a general multiplication.
This patch improves that fallback by reusing expand_mult if
can_create_pseudo_p, rather than emitting a MULT pattern directly.

In order to increase the chances of being able to use a simple
add-and-shift, the patch also tries to compute VG * the lowest set
bit of the multiplier, rather than always using CNTD as the basis
for the multiplication path.

This is tested by the ACLE patches but is really an independent
improvement.

2019-08-15  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_add_offset): In the fallback
	multiplication case, try to compute VG * (lowest set bit) directly
	rather than always basing the multiplication on VG.  Use
	expand_mult for the multiplication if we can.

gcc/testsuite/
	* gcc.target/aarch64/sve/loop_add_4.c: Expect 10 INCWs and
	INCDs rather than 8.

From-SVN: r274519