Reject really weird coalescer case when trying to merge identical subregisters

of different register classes. e.g.

  %reg1048:3<def> = EXTRACT_SUBREG %RAX<kill>, 3

Where %reg1048 is a GR32 register. This is not impossible to handle, but it is
pretty hard and very rare.

This should unbreak the dragonegg builder.

llvm-svn: 102672