PowerPC: Mark super regs of reserved regs reserved.

When a register like R1 is reserved, X1 should be reserved as well. This
was already done "manually" when 64bit code was enabled, however using
the markSuperRegs() function on the base register is more convenient and
allows to use the checksAllSuperRegsMarked() function even in 32bit mode
to avoid accidental breakage in the future.

This is also necessary to allow https://reviews.llvm.org/D28881

Differential Revision: https://reviews.llvm.org/D29056

llvm-svn: 292870