Add TRI::getCommonSuperRegClass().

This function is a generalization of getMatchingSuperRegClass() to the
symmetric case where both sides are using a sub-register index. It will
find a super-register class and sub-register indexes that make this
diagram commute:

                                   PreA
                       SuperRC  ---------->  RCA

                          |                   |
                          |                   |
                     PreB |                   | SubA
                          |                   |
                          |                   |
                          V                   V

                         RCB    ----------> SubRC
                                   SubB

This can be used to coalesce copies like:

  %vreg1:sub16 = COPY %vreg2:sub16; GR64:%vreg1, GR32: %vreg2

llvm-svn: 156317