Implement two virtual functions in SPUTargetLowering.

Before the implementation of isLegalAddressingMode, some rare cases 
of code were miscompiled if optimized with the LoopStrengthReduce pass.
It is unclear (to me) if LSR is "allowed" to produce wrong code with a 
bad TargetLowering, or if the bug is elsewhere and this patch just 
hides it.

llvm-svn: 115919