Slightly change TableGen's definition of a register subclass.

A subclass is allowed to have a larger spill size than the superclass, and the
spill alignment must be a multiple of the superclass alignment. This causes
the following new subclass relations:

=== Alpha ===
F4RC -> F8RC

=== PPC ===
F4RC -> F8RC

=== SPU ===
R8C -> R16C -> R32C/R32FP -> R64C/R64FP -> GPRC/VECREG

=== X86 ===
FR32  -> FR64  -> VR128
RFP32 -> RFP64 -> RFP80

These subclass relations are consistent with the behaviour of -join-cross-class-copies.

llvm-svn: 70511