son of subreg rtl documentation

[Paolo Bonzini]

Here's my take on the part on mems, trying to make the usage in s390 
sound less brittle (because cleanups in the area will make it kosher, 
not invalid).

Paolo



@code{subreg}s of @code{mem} can be used in instruction patterns freely, 
however care needs to be put because such @code{subreg}s are considered 
to be @code{register_operand}s rather than @code{memory_operand}s before 
and during reload.  Therefore, if the @code{subreg}s are moved to 
another instructions, they will need not be replaced by plain
@code{mem}s until the reload pass.

On machines that do not do instruction scheduling, combine may also 
create @code{subreg}s of @code{mem}.  For machine that do instruction 
scheduling, the need for reloads will introduce inaccurate scheduling, 
and so @code{subreg}s of @code{mem} should never be used.  Future 
versions of GCC may not allow @code{subreg}s of @code{mem} to be 
considered @code{register_operand}s.  To prepare for this, 
@code{subreg}s of @code{mem} (if needed) should be spelled explicitly in 
the machine description as @code{subreg}s of a @code{memory_operand}.