Arm: Add support for auto-vectorization using HF mode.

Arm: Add support for auto-vectorization using HF mode.

This adds support to the auto-vectorizer to support HFmode vectorization for
AArch32.  This is supported when +fp16 is used.  I wonder if I should disable
the returning of the type if the option isn't enabled.

At the moment it will be returned but the vectorizer will try and fail to use
it.  It wastes a few compile cycles but doesn't result in bad code.

gcc/ChangeLog:

	* config/arm/arm.c (arm_preferred_simd_mode): Add E_HFmode.

gcc/testsuite/ChangeLog:

	* gcc.target/arm/vect-half-floats.c: New test.