Use IFN_SQRT in tree-vect-patterns.c

Use IFN_SQRT in tree-vect-patterns.c

In practice all targets that can vectorise sqrt define the appropriate
sqrt<mode>2 optab.  The only case where this isn't immediately obvious
is the libmass support in rs6000.c, but Mike Meissner said that it shouldn't
be exercised for sqrt.

This patch therefore uses the internal function interface instead of
going via the target hook.

Tested on x86_64-linux-gnu, aarch64-linux-gnu, arm-linux-gnueabi and
powerpc64-linux-gnu.

gcc/
	* tree-vect-patterns.c: Include internal-fn.h.
	(vect_recog_pow_pattern): Use IFN_SQRT instead of BUILT_IN_SQRT*.

From-SVN: r230490