nv50: use larger arrays to silence warnings and fix buffer overflows

nv50: use larger arrays to silence warnings and fix buffer overflows

The warnings were:
nv50_pc_regalloc.c: In function ‘pass_generate_phi_movs’:
nv50_pc_regalloc.c:423:41: warning: array subscript is above array bounds
codegen/nv50_ir_peephole.cpp: In member function ‘bool nv50_ir::MemoryOpt::replaceStFromSt(nv50_ir::Instruction*, nv50_ir::MemoryOpt::Record*)’:
codegen/nv50_ir_peephole.cpp:1475:18: warning: array subscript is above array bounds
codegen/nv50_ir_peephole.cpp:1475:18: warning: array subscript is above array bounds
codegen/nv50_ir_peephole.cpp:1475:18: warning: array subscript is above array bounds
codegen/nv50_ir_peephole.cpp:1475:18: warning: array subscript is above array bounds

And add some assertions to catch this sooner in debug builds.