[AArch64] Add SVE mul_highpart patterns

[AArch64] Add SVE mul_highpart patterns

One advantage of the new permute handling compared to the old way is
that we can now easily take advantage of the vectoriser's divmod patterns
for SVE.

2018-03-13  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* config/aarch64/iterators.md (UNSPEC_SMUL_HIGHPART)
	(UNSPEC_UMUL_HIGHPART): New constants.
	(MUL_HIGHPART): New int iteraor.
	(su): Handle UNSPEC_SMUL_HIGHPART and UNSPEC_UMUL_HIGHPART.
	* config/aarch64/aarch64-sve.md (<su>mul<mode>3_highpart): New
	define_expand.
	(*<su>mul<mode>3_highpart): New define_insn.

gcc/testsuite/
	* gcc.target/aarch64/sve/mul_highpart_1.c: New test.
	* gcc.target/aarch64/sve/mul_highpart_1_run.c: Likewise.

From-SVN: r258487