[RegisterCoalescer] Remove copies to reserved registers

This allows the RegisterCoalescer to join "non-flipped" range pairs with a
physical destination register -- which allows the RegisterCoalescer to remove
copies like this:

<vreg> = something (maybe a load, for example)
... (things that don't use PHYSREG)
PHYSREG = COPY <vreg>

(with all of the restrictions normally applied by the RegisterCoalescer: having
compatible register classes, etc. )

Previously, the RegisterCoalescer handled only the opposite case (copying
*from* a physical register). I don't handle the problem fully here, but try to
get the common case where there is only one use of <vreg> (the COPY).

An upcoming commit to the PowerPC backend will make this pattern much more
common on PPC64/ELF systems.

llvm-svn: 226071