arm.h (BASE_REG_CLASS): Always return LO_REGS for Thumb.

* arm.h (BASE_REG_CLASS): Always return LO_REGS for Thumb.
(MODE_BASE_REG_CLASS, case Thumb): Only return BASE_REGS if we know
that we have a SImode access, and only then if reload hasn't completed;
for all other cases, use LO_REGS.

From-SVN: r57644