[AArch64] Emit square root using the Newton series

[AArch64] Emit square root using the Newton series

2016-06-13  Evandro Menezes  <e.menezes@samsung.com>
            Wilco Dijkstra  <wilco.dijkstra@arm.com>

gcc/
	* config/aarch64/aarch64-protos.h
	(aarch64_emit_approx_rsqrt): Replace with new function
	"aarch64_emit_approx_sqrt".
	(cpu_approx_modes): New member "sqrt".
	* config/aarch64/aarch64.c
	(generic_approx_modes): New member "sqrt".
	(exynosm1_approx_modes): Likewise.
	(xgene1_approx_modes): Likewise.
	(aarch64_emit_approx_rsqrt): Replace with new function
	"aarch64_emit_approx_sqrt".
	(aarch64_override_options_after_change_1): Handle new option.
	* config/aarch64/aarch64-simd.md
	(rsqrt<mode>2): Use new function instead.
	(sqrt<mode>2): New expansion and insn definitions.
	* config/aarch64/aarch64.md: Likewise.
	* config/aarch64/aarch64.opt
	(mlow-precision-sqrt): Add new option description.
	* doc/invoke.texi (mlow-precision-sqrt): Likewise.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@237396 138bc75d-0d04-0410-961f-82ee72b054a4