Add TRI::getSubClassWithSubReg(RC, Idx) function.

This function is used to constrain a register class to a sub-class that
supports the given sub-register index.

For example, getSubClassWithSubReg(GR32, sub_8bit) -> GR32_ABCD.

The function will be used to compute register classes when emitting
INSERT_SUBREG and EXTRACT_SUBREG nodes and for register class inflation
of sub-register operations.

The version provided by TableGen is usually adequate, but targets can
override.

llvm-svn: 141142