[arm] PR target/82975: Guard against reg_renumber being NULL in arm.h

[arm] PR target/82975: Guard against reg_renumber being NULL in arm.h

In this bug we ICE when checking REGNO_OK_FOR_INDEX_P on arm during pre-IRA scheduling.
This is because REGNO_OK_FOR_INDEX_P ends up checking the reg_renumber array.
Before IRA reg_renumber is NULL and thus we segfault.

The fix is to guard the use of reg_renumber in the logic in TEST_REGNO in arm.h.
On aarch64, for example, we also guard against the reg_renumber == NULL case.
This fixes the ICE. I also remove the part of the comment that muses on when reg_renumber
is available as with this patch it should now be safe to use at any point.

Bootstrapped and tested on arm-none-linux-gnueabihf.

	PR target/82975
	* config/arm/arm.h (TEST_REGNO): Check reg_renumber is set before
	accessing it.  Adjust comment.

	* gcc.dg/pr82975.c: New test.

From-SVN: r255830