gcc/lower_subreg.c: Prevent decomposition if modes are not tieable

gcc/lower_subreg.c: Prevent decomposition if modes are not tieable

Preventing decomposition if modes are not tieable is necessary to
stop AArch64 partial Neon structure modes being treated as packed in
registers.

This is a necessary prerequisite for a future AArch64 PCS change to
maintain good code generation.

gcc/ChangeLog:

2021-10-14  Jonathan Wright  <jonathan.wright@arm.com>

	* lower-subreg.c (simple_move): Prevent decomposition if
	modes are not tieable.