RegisterCoalscer: Only coalesce complete reserved registers.

The coalescer eliminates copies from reserved registers of the form:
   %vregX = COPY %rY
in the case where %rY is a reserved register. However this turns out to
be invalid if only some of the subregisters are reserved (see also
https://reviews.llvm.org/D26648).

Differential Revision: https://reviews.llvm.org/D26687

llvm-svn: 288428