PR82809: register handling in ix86_vector_duplicate_value

PR82809: register handling in ix86_vector_duplicate_value

When adding the call to gen_vec_duplicate, I failed to notice that
code further down modified the VEC_DUPLICATE in place.  That isn't
safe if gen_vec_duplicate returned a const_vector.

2017-11-02  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	PR target/82809
	* config/i386/i386.c (ix86_vector_duplicate_value): Use
	gen_vec_duplicate after forcing the scalar into a register.

gcc/testsuite/
	* gcc.dg/pr82809.c: New test.

From-SVN: r254366