Handle a null lhs in expand_direct_optab_fn (PR85862)

Handle a null lhs in expand_direct_optab_fn (PR85862)

This PR showed that the normal function for expanding directly-mapped
internal functions didn't handle the case in which the call was only
being kept for its side-effects.

2018-05-22  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR middle-end/85862
	* internal-fn.c (expand_direct_optab_fn): Cope with a null lhs.

gcc/testsuite/
	PR middle-end/85862
	* gcc.dg/torture/pr85862.c: New test.

From-SVN: r260504