Simplify INSERT_SUBREG emission.

The register class created by INSERT_SUBREG and SUBREG_TO_REG must be
legal and support the SubIdx sub-registers.

The new getSubClassWithSubReg() hook can compute that.

This may create INSERT_SUBREG instructions defining a larger register
class than the sub-register being inserted.  That is OK,
RegisterCoalescer will constrain the register class as needed when it
eliminates the INSERT_SUBREG instructions.

llvm-svn: 141198