Change SPU register re-interpretations from OR to COPY_TO_REGCLASS instruction.

This cleans up after the mess r108567 left in the CellSPU backend.
ORCvt-instruction were used to reinterpret registers, and the ORs were then
removed by isMoveInstr(). This patch now removes 350 instrucions of format:
	or $3, $3, $3
(from the 52 testcases in CodeGen/CellSPU). One case of a nonexistant or is
checked for.

Some moves of the form 'ori $., $., 0' and 'ai $., $., 0' still remain.

llvm-svn: 114074