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

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

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

I ended up doing this much later than originally intended, sorry...

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

gcc/
	* config/aarch64/aarch64.c (aarch64_emit_mult): New function.
	(aarch64_emit_approx_div): Add SVE support.  Use aarch64_emit_mult
	instead of emitting multiplication instructions directly.
	* config/aarch64/iterators.md (SVE_COND_FP_BINARY_OPTAB): New iterator.
	* config/aarch64/aarch64-sve.md (div<mode>3, @aarch64_frecpe<mode>)
	(@aarch64_frecps<mode>): New expanders.

gcc/testsuite/
	* gcc.target/aarch64/sve/recip_1.c: New test.
	* gcc.target/aarch64/sve/recip_1_run.c: Likewise.
	* gcc.target/aarch64/sve/recip_2.c: Likewise.
	* gcc.target/aarch64/sve/recip_2_run.c: Likewise.