Tidy store_bit_field_1 & co.

insv, extv and extzv have an unusual interface: the structure operand is
supposed to have word_mode if stored in registers or byte_mode if stored
in memory.  Andrew's patch to try different insv modes:

   http://gcc.gnu.org/ml/gcc-patches/2012-09/msg00126.html

prompted me to try making the patterns more like other optabs.

The use of word and byte "units" for registers and memory respectively
is pretty deeply engrained into the current expand routines, even in the
parts that don't deal directly with the .md patterns.  E.g. the bitnum
parameter to store_bit_field always counts from the "leftmost" bit of OP0,
but store_bit_field_1 internally converts it to a trio of "unit",
"offset" (number of whole units) and "bitpos" (position within a unit).
The latter two are then also used in the interface to store_fixed_bit_field,
with the unit being implicit.  store_split_bit_field uses the original
"bitnum"-style parameter instead.

This patch makes the code use the original "bitnum" throughout,
and only separate into units where locally useful.

Also, if the field spans two words of a register OP0, store_bit_field_1
reduces OP0 to just the first word.   It then makes sure that we fall
through to store_fixed_bit_field, which in turn calls store_split_bit_field,
which knows that OP0 is only partial.  I think this is dangerous:
it's the only time that store_bit_field_1 trims OP0 to cover only
part of the field, and so adds another special case for the rest
of the function to handle and ignore.  It also makes the interface
to store_fixed_bit_field more complicated.

The patch instead makes store_bit_field_1 call store_split_bit_field
directly where appropriate.

diffstat for this patch and the one I'm about to post says:

 expmed.c |  640 +++++++++++++++++++++++++--------------------------------------
 1 file changed, 261 insertions(+), 379 deletions(-)

so I'd like to submit them as "clean ups" regardless of whether
I ever get around to the main patterns change.

The patch is probably quite hard to review, sorry.  I've made the changelog
a bit more detailed than usual in order to list the individual points.

Tested on x86_64-linux-gnu, powerpc64-linux-gnu, mipsisa64-elf (both -EL
and -EB) and mipsisa32-elf (also both -EL and -EB).  OK to install?

Richard

gcc/
	* expmed.c (store_bit_field_1): Remove unit, offset, bitpos and
	byte_offset from the outermost scope.  Express conditions in terms
	of bitnum rather than offset, bitpos and byte_offset.  Split the
	plain move cases into two, one for memory accesses and one for
	register accesses.  Allow simplify_gen_subreg to fail rather
	than calling validate_subreg.  Move the handling of multiword
	OP0s after the code that coerces VALUE to an integer mode.
	Use simplify_gen_subreg for this case and assert that it succeeds.
	If the field still spans several words, pass it directly to
	store_split_bit_field.  Assume after that point that both sources
	and register targets fit within a word.  Replace x-prefixed
	variables with non-prefixed forms.  Compute the bitpos for insv
	register operands directly in the chosen unit size, rather than
	going through an intermediate BITS_PER_WORD unit size.
	Update the call to store_fixed_bit_field.
	(store_fixed_bit_field): Replace the bitpos and offset parameters
	with a single bitnum parameter, of the same form as store_bit_field.
	Assume that OP0 contains the full field.  Simplify the memory offset
	calculation.  Assert that the processed OP0 has an integral mode.
	(store_split_bit_field): Update the call to store_fixed_bit_field.

Tidy store_bit_field_1 & co.

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