Hi,

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.

Bootstrapped and regression tested on:
* x86_64-pc-linux-gnu - no issues.
* aarch64-none-linux-gnu - two test failures which will be fixed by
  the next patch in this series. 

Ok for master?

Thanks,
Jonathan

---

gcc/ChangeLog:

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

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