gcc/expmed.c: Ensure vector modes are tieable before extraction

gcc/expmed.c: Ensure vector modes are tieable before extraction

Extracting a bitfield from a vector can be achieved by casting the
vector to a new type whose elements are the same size as the desired
bitfield, before generating a subreg. However, this is only an
optimization if the original vector can be accessed in the new
machine mode without first being copied - a condition denoted by the
TARGET_MODES_TIEABLE_P hook.

This patch adds a check to make sure that the vector modes are
tieable before attempting to generate a subreg. This is a necessary
prerequisite for a subsequent patch that will introduce new machine
modes for Arm Neon vector-tuple types.

gcc/ChangeLog:

2021-10-11  Jonathan Wright  <jonathan.wright@arm.com>

	* expmed.c (extract_bit_field_1): Ensure modes are tieable.