Ada mainline bootstrap failing on x86 and x86_64

[Daniel Berlin]

> I get the same failure on powerpc-darwin plus now I also get the following
> related failures with the C compiler:
> FAIL: gcc.c-torture/execute/20010123-1.c compilation,  -O1
> FAIL: gcc.c-torture/execute/20010123-1.c compilation,  -O2
> FAIL: gcc.c-torture/execute/20010123-1.c compilation,  -O3 
> -fomit-frame-pointer
> FAIL: gcc.c-torture/execute/20010123-1.c compilation,  -O3 
> -fomit-frame-pointer -funroll-loops
> FAIL: gcc.c-torture/execute/20010123-1.c compilation,  -O3 
> -fomit-frame-pointer -funroll-all-loops -finline-functions
> FAIL: gcc.c-torture/execute/20010123-1.c compilation,  -O3 -g
> FAIL: gcc.c-torture/execute/20010123-1.c compilation,  -Os
>

I tested this patch on powerpc-darwin only a few days ago, and wasn't 
getting this failure.
Sigh.

>
> We are removing a modify expression which is not dead at all.

This patch shouldn't change what gets marked by DCE as dead, unless 
something was wrong to begin with.
Renaming errors are one thing (which is what the Ada failure looks like), 
but removing code that wasn't removed before shouldn't be happening 
because of the patch.