emit-rtl: Use simplify_subreg_regno to validate hardware subregs [PR119966]

emit-rtl: Use simplify_subreg_regno to validate hardware subregs [PR119966]

PR119966 showed that combine could generate unfoldable hardware subregs
for pru-unknown-elf.  To fix, strengthen the checks performed by
validate_subreg.

The simplify_subreg_regno performs more validity checks than
the simple info.representable_p.  Most importantly, the
targetm.hard_regno_mode_ok hook is called to ensure the hardware
register is valid in subreg's outer mode.  This fixes the rootcause
for PR119966.

The checks for stack-related registers are bypassed because the i386
backend generates them, in this seemingly valid peephole optimization:

   ;; Attempt to always use XOR for zeroing registers (including FP modes).
   (define_peephole2
     [(set (match_operand 0 "general_reg_operand")
           (match_operand 1 "const0_operand"))]
     "GET_MODE_SIZE (GET_MODE (operands[0])) <= UNITS_PER_WORD
      && (! TARGET_USE_MOV0 || optimize_insn_for_size_p ())
      && peep2_regno_dead_p (0, FLAGS_REG)"
     [(parallel [(set (match_dup 0) (const_int 0))
                 (clobber (reg:CC FLAGS_REG))])]
     "operands[0] = gen_lowpart (word_mode, operands[0]);")

Testing done:
  * No regressions were detected for C and C++ on x86_64-pc-linux-gnu.
  * "contrib/compare-all-tests i386" showed no difference in code
    generation.
  * No regressions for pru-unknown-elf.
  * Reverted r16-809-gf725d6765373f7 to expose the now latent PR119966.
    Then ensured pru-unknown-elf build is ok.  Only two cases regressed
    where rnreg pass transforms a valid hardware subreg into invalid
    one.  But I think that is not related to combine's PR119966:
      gcc.c-torture/execute/20040709-1.c
      gcc.c-torture/execute/20040709-2.c

	PR target/119966

gcc/ChangeLog:

	* emit-rtl.cc (validate_subreg): Call simplify_subreg_regno
	instead of checking info.representable_p..
	* rtl.h (simplify_subreg_regno): Add new argument
	allow_stack_regs.
	* rtlanal.cc (simplify_subreg_regno): Do not reject
	stack-related registers if allow_stack_regs is true.

Co-authored-by: Richard Sandiford <richard.sandiford@arm.com>
Co-authored-by: Andrew Pinski <quic_apinski@quicinc.com>
Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>