[patch, arm] Prefer LO_REG registers in regrename

[Yao Qi]

This patch is to implement to targethook preferred_rename_class, in
order to prefer LO_REGS registers over GENERAL_REGS registers in Thumb-2.

EEMBC doesn't show speed improvements, but code size of some benchmarks
in EEMBC is reduced by 0.1% ~ 0.2%.  Regression tested on '2010-11-30'
trunk.  OK for mainline?

-- 
Yao (齐尧)
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