emit-rtl: Fix a -fcompare-debug issue due to var-tracking [PR103808]

We get a -fcompare-debug FAIL on the following testcase.  The problem is
that during cprop we get when a TImode pseudo holding x is being
constructed:
(debug_insn 111 59 103 7 (var_location:TI D#2 (clobber (const_int 0 [0]))) -1
     (nil))
(insn 103 111 110 7 (clobber (reg/v:TI 89 [ x ])) "pr103808.c":8:9 -1
     (nil))
(debug_insn 110 103 104 7 (var_location:TI D#2 (subreg:TI (reg:DI 111 [ x ]) 0)) -1
     (nil))
(insn 104 110 109 7 (set (subreg:DI (reg/v:TI 89 [ x ]) 0)
        (reg:DI 111 [ x ])) "pr103808.c":8:9 80 {*movdi_internal}
     (expr_list:REG_DEAD (reg:DI 111 [ x ])
        (nil)))
Now, during RA that paradoxical subreg in a debug insn obviously can't
affect where pseudo 111 is allocated and RA puts it into the bp register,
so we have:
(debug_insn 110 111 109 4 (var_location:TI D#2 (reg:TI 6 bp [orig:111 x ] [111])) -1
     (nil))
Now, during var-tracking when we for:
(debug_insn 25 23 26 3 (var_location:TI x (concatn/v:TI [
            (reg:DI 6 bp [orig:111 x ] [111])
            (subreg:DI (debug_expr:TI D#2) 8)
        ])) "pr103808.c":8:9 -1
     (nil))
try to simplify the highpart subreg of bp, gen_rtx_REG_offset is called in:
      if (HARD_REGISTER_NUM_P (final_regno))
        {
          rtx x = gen_rtx_REG_offset (op, outermode, final_regno,
                                      subreg_memory_offset (outermode,
                                                            innermode, byte));
and that unfortunately sets REG_ATTRS on stack_pointer_rtx, because
gen_rtx_REG_offset uses gen_rtx_REG which for Pmode STACK_POINTER_REGNUM
returns stack_pointer_rtx rather than newly created register.
The clobbering of REG_ATTRS on the shared stack_pointer_rtx then shows up
in the dumps as (reg/f:DI 7 sp [ x+8 ]) instead of (reg/f:DI 7 sp)
that shows up without var-tracking.
Clobbering of REG_ATTRS on the shared *_pointer_rtx looks just wrong.
So, IMHO either simplify_gen_subreg -> gen_rtx_REG_offset should call
gen_raw_REG to make sure we get a new non-shared REG we can set REG_ATTRS
on, or we should make sure that we don't overwrite the REG_ATTRS on the
shared REGs (but then simplify_gen_subreg shouldn't try to overwrite
ORIGINAL_REGNO on those either).
For non-DEBUG_INSNs, I'd hope this never happens, the RA shouldn't allocate
multi-word regs overlapping with stack pointer, hard frame pointer etc.

2021-12-30  Jakub Jelinek  <jakub@redhat.com>

	PR debug/103808
	* emit-rtl.c (gen_rtx_REG_offset): Use gen_raw_REG instead of
	gen_rtx_REG.

	* gcc.dg/pr103808.c: New test.