aarch64: Add SVE support for -mlow-precision-sqrt

aarch64: Add SVE support for -mlow-precision-sqrt

SVE was missing support for -mlow-precision-sqrt, which meant that
-march=armv8.2-a+sve -mlow-precision-sqrt could cause a performance
regression compared to -march=armv8.2-a -mlow-precision-sqrt.

2020-02-21  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_emit_approx_sqrt): Add SVE
	support.  Use aarch64_emit_mult instead of emitting multiplication
	instructions directly.
	* config/aarch64/aarch64-sve.md (sqrt<mode>2, rsqrt<mode>2)
	(@aarch64_rsqrte<mode>, @aarch64_rsqrts<mode>): New expanders.

gcc/testsuite/
	* gcc.target/aarch64/sve/rsqrt_1.c: New test.
	* gcc.target/aarch64/sve/rsqrt_1_run.c: Likewise.
	* gcc.target/aarch64/sve/sqrt_1.c: Likewise.
	* gcc.target/aarch64/sve/sqrt_1_run.c: Likewise.