[PR rtl-optimization/116136] Fix previously latent SUBREG simplification bug

This fixes a testsuite regression seen on m68k after some of the recent ext-dce
changes.  Ultimately Richard S and I have concluded the bug was a latent issue
in subreg simplification.

Essentially when simplifying something like

(set (target:M1) (subreg:M1 (subreg:M2 (reg:M1) 0) 0))

Where M1 > M2.  We'd simplify to:

(set (target:M1) (reg:M1))

The problem is on a big endian target that's wrong.   Consider if M1 is DI and
M2 is SI.    The original should extract bits 32..63 from the source register
and store them into bits 0..31 of the target register. In the simplified form
it's just a copy, so bits 0..63 of the source end up bits 0..63 of the target.

This shows up as the following regressions on the m68k:

> Tests that now fail, but worked before (3 tests):
>
> gcc: gcc.c-torture/execute/960416-1.c   -O2  execution test
> gcc: gcc.c-torture/execute/960416-1.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  execution test
> gcc: gcc.c-torture/execute/960416-1.c   -Os  execution test

The fix is pretty trivial, instead of hardcoding "0" as the byte offset in the
test for the simplification, instead we need to use the subreg_lowpart_offset.

Anyway, bootstrapped and regression tested on m68k and x86_64 and tested on the
other embedded targets as well without regressions.  Naturally it fixes the
regression noted above.  I haven't see other testsuite improvements when I spot
checked some of the big endian crosses.

	PR rtl-optimization/116136
gcc/
	* simplify-rtx.cc (simplify_context::simplify_subreg): Check
	that we're working with the lowpart offset rather than byte 0.