AArch64: decode all MRS/MSR forms early to avoid saving FeatureBits.

Currently, we're adding a uint64_t describing the current subtarget so
that matching can check whether the specified register is valid.
However, we want to move to a bitset for those bits (x86 has more than
64 of them).

This can't live in a union so it's probably better to do the checks
early (especially as there are only 3 of them).

llvm-svn: 226841