Insert class IceInstX8632 between IceInstX8632* and IceInstTarget.

This makes it more practical to add X86-specific stuff that applies to
all x86 target instructions.
Add llvm::isa<> support for low-level target instructions.

This is slightly fragile because in the future e.g. the IceInstX8632
Kind space will overlap with the IceInstX8664 space.  That shouldn't
really be a problem though since instructions from different targets
won't coexist in the same Cfg.