[PATCH 0/5] Fix handling of word subregs of wide registers

[Richard Sandiford <richard dot sandiford at arm dot com>]

This series is a cleaned-up version of:

    https://gcc.gnu.org/ml/gcc/2014-03/msg00163.html

The underlying problem is that the semantics of subregs depend on the
word size.  You can't have a subreg for byte 2 of a 4-byte word, say,
but you can have a subreg for word 2 of a 4-word value (as well as lowpart
subregs of that word, etc.).  This causes problems when an architecture has
wider-than-word registers, since the addressability of a word can then depend
on which register class is used.

The register allocators need to fix up cases where a subreg turns out to
be invalid for a particular class.  This is really an extension of what
we need to do for CANNOT_CHANGE_MODE_CLASS.

Tested on x86_64-linux-gnu, powerpc64-linux-gnu and aarch64_be-elf.

Thanks,
Richard