Remove custom allocation order boilerplate that is no longer needed.

The register allocators automatically filter out reserved registers and
place the callee saved registers last in the allocation order, so custom
methods are no longer necessary just for that.

Some targets still use custom allocation orders:

ARM/Thumb: The high registers are removed from GPR in thumb mode. The
NEON allocation orders prefer to use non-VFP2 registers first.

X86: The GR8 classes omit AH-DH in x86-64 mode to avoid REX trouble.

SystemZ: Some of the allocation orders are omitting R12 aliases without
explanation. I don't understand this target well enough to fix that. It
looks like all the boilerplate could be removed by reserving the right
registers.

llvm-svn: 132781