Except for one case, get_vectype_for_scalar_type_and_size calculates
what the vector mode should be and then calls build_vector_type,
which recomputes the mode from scratch.  This patch makes it use
build_vector_type_for_mode instead.

The exception mentioned above is when preferred_simd_mode returns
an integer mode, which it does if no appropriate vector mode exists.
The integer mode in question is usually word_mode, although epiphany
can return a doubleword mode in some cases.

There's no guarantee that this integer mode is appropriate, since for
example the scalar type could be a float.  The traditional behaviour is
therefore to use the integer mode to determine a size only, and leave
mode_for_vector to pick the TYPE_MODE.  (Note that it can actually end
up picking a vector mode if the target defines a disabled vector mode.
We therefore still need to check TYPE_MODE after building the type.)

2019-11-14  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-stmts.c (get_vectype_for_scalar_type_and_size): If
	targetm.vectorize.preferred_simd_mode returns an integer mode,
	use mode_for_vector to decide what the vector type's mode
	should actually be.  Use build_vector_type_for_mode instead
	of build_vector_type.

From-SVN: r278234