[trunk] Addition to subreg section of rtl.text.

[Richard Kenner]

> It is seldom necessary to wrap hard registers in @code{subreg}s;
> such registers would normally reduce to a single @code{reg} rtx.

Are these valid?  I know we've gone back and forth, but I thought the
current position is that SUBREGs of hard regs are only allowed
transitorily (e.g., during reload), but must be converted into a hard
reg by the end of the pass.