Add missing check for BUILT_IN_MD (PR 91444)

Add missing check for BUILT_IN_MD (PR 91444)

In this PR we were passing an ordinary non-built-in function to
targetm.vectorize.builtin_md_vectorized_function, which is only
supposed to handle BUILT_IN_MD.

2019-08-15  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR middle-end/91444
	* tree-vect-stmts.c (vectorizable_call): Check that the function
	is a BUILT_IN_MD function before passing it to
	targetm.vectorize.builtin_md_vectorized_function.

From-SVN: r274524