[committed] Provide patterns for signed bitfield extractions on H8

Inspired by Roger's work on the ARC port, this patch provides a
define_and_split pattern to optimize sign extended bitfields starting at
position 0 using an approach that doesn't require shifting.

It then builds on that to provide another define_and_split pattern to support
arbitrary signed bitfield extractions -- it uses a right logical shift to move
the bitfield into position 0, then the specialized pattern above to sign extend
the MSB of the field through the rest of the register.

This is often, but certainly not always, better than a two shift approach.  The
code uses the sizes of the sequences to select between the two shift approach
and single shift with extension from an arbitrary location approach.

There's certainly further improvements that could be made here, but I think
we're getting the bulk of the improvements already.

Regression tested on the H8 port without errors.  Installing on the trunk.

gcc/
	* config/h8300/h8300-protos.h (use_extvsi): Prototype.
	* config/h8300/combiner.md: Two new define_insn_and_split patterns
	to implement signed bitfield extractions.
	* config/h8300/h8300.cc (use_extvsi): New function.

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