[PATCH] Make avoid_constant_pool_reference smarter

[Paolo Bonzini]

This patch moves one case from CSE's fold_rtx_mem to 
avoid_constant_pool_reference.  CSE can extract the low part of a 
constant that resides in the pool, or a single word of a multiword 
constant.  In a slightly cleaner way, a_c_p_r uses simplify_subreg to 
convert constants between different modes.  This patch equips a_c_p_r to 
support offsetted addresses (e.g. to extract words from multiword 
constants).

In CSE's fold_rtx_mem, the part dealing with LABEL_REF (basically 
performing a kind of jump threading) is practically dead and was toggles 
even less frequently since Alexandre Oliva fixed PR18628 (the dead-label 
bug).  So, fold_rtx_mem could reduce to 
find_best_addr+avoid_constant_pool_reference.  And similarly, 
equiv_constant's MEM case could be reduced to 
avoid_constant_pool_reference+lookup.

These however are follow-ups that cannot happen in the 4.1 frame (and 
I'm thinking more and more about a branch...).

Bootstrapped/regtested all languages on powerpc-apple-darwin7, ok for 
mainline?

Paolo
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