emit-rtl: Allow extra checks for paradoxical subregs [PR119966]

emit-rtl: Allow extra checks for paradoxical subregs [PR119966]

When a paradoxical subreg is detected, validate_subreg exits early, thus
skipping the important checks later in the function.

Fix by continuing with the checks instead of declaring early that the
paradoxical subreg is valid.

One of the newly allowed subsequent checks needed to be disabled for
paradoxical subregs.  It turned out that combine attempts to create
a paradoxical subreg of mem even for strict-alignment targets.
That is invalid and should eventually be rejected, but is
temporarily left allowed to prevent regressions for
armv8l-unknown-linux-gnueabihf.  See PR120329 for more details.

Tests I did:
 - No regressions were found for C and C++ for the following targets:
   - native x86_64-pc-linux-gnu
   - cross riscv64-unknown-linux-gnu
   - cross riscv32-none-elf
 - Sanity checked armv8l-unknown-linux-gnueabihf by cross-building
   up to including libgcc.  Linaro CI bot further confirmed there
   are no regressions.
 - Sanity checked powerpc64-unknown-linux-gnu by building native
   toolchain, but I could not setup qemu-user for DejaGnu testing.

	PR target/119966

gcc/ChangeLog:

	* emit-rtl.cc (validate_subreg): Do not exit immediately for
	paradoxical subregs.  Filter subsequent tests which are
	not valid for paradoxical subregs.

Co-authored-by: Richard Sandiford <richard.sandiford@arm.com>
Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>