tree-optimization/114921 - _Float16 -> __bf16 isn't noop

tree-optimization/114921 - _Float16 -> __bf16 isn't noop

The vectorizer handles a _Float16 to __bf16 conversion through
vectorizable_assignment, thinking it's a noop.  The following
fixes this by requiring the same vector component mode when
checking for CONVERT_EXPR_CODE_P, being stricter than for
VIEW_CONVERT_EXPR.

	PR tree-optimization/114921
	* tree-vect-stmts.cc (vectorizable_assignment): Require
	same vector component modes for input and output for
	CONVERT_EXPR_CODE_P.