[AArch64] Use SVE reversed shifts in preference to MOVPRFX

[AArch64] Use SVE reversed shifts in preference to MOVPRFX

This patch makes us use reversed SVE shifts when the first operand
can't be tied to the output but the second can.  This is tested
more thoroughly by the ACLE patches but is really an independent
improvement.

2019-08-15  Richard Sandiford  <richard.sandiford@arm.com>
	    Prathamesh Kulkarni  <prathamesh.kulkarni@linaro.org>

gcc/
	* config/aarch64/aarch64-sve.md (*v<ASHIFT:optab><SVE_I:mode>3):
	Add an alternative that uses reversed shifts.

gcc/testsuite/
	* gcc.target/aarch64/sve/shift_1.c: Accept reversed shifts.

Co-Authored-By: Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>
From-SVN: r274512