rtl: relax validate_subreg to allow paradoxical subregs that change mode

rtl: relax validate_subreg to allow paradoxical subregs that change mode

This patch relaxes the subreg invariant that you can only change modes
or make it paradoxical in one conversion. i.e. it now allows subreg:V2DI (reg:DF ..))

This is well defined in the generic sense and allowing it would enable
you to write RTL without the extra moves which can be interfered with by
combine.

Patch has been pre-approved[1], but giving people chance to object

[1] https://gcc.gnu.org/pipermail/gcc-patches/2023-September/629119.html

gcc/ChangeLog:

	* emit-rtl.cc (validate_subreg): Relax subreg rule.