middle-end: Allow backend to expand/split double word compare to 0/-1.

This patch to the middle-end's RTL expansion reorders the code in
emit_store_flag_1 so that the backend has more control over how best
to expand/split double word equality/inequality comparisons against
zero or minus one.  With the current implementation, the middle-end
always decides to lower this idiom during RTL expansion using SUBREGs
and word mode instructions, without ever consulting the backend's
machine description.  Hence on x86_64, a TImode comparison against zero
is always expanded as:

(parallel [
  (set (reg:DI 91)
       (ior:DI (subreg:DI (reg:TI 88) 0)
               (subreg:DI (reg:TI 88) 8)))
  (clobber (reg:CC 17 flags))])
(set (reg:CCZ 17 flags)
     (compare:CCZ (reg:DI 91)
                  (const_int 0 [0])))

This patch, which makes no changes to the code itself, simply reorders
the clauses in emit_store_flag_1 so that the middle-end first attempts
expansion using the target's doubleword mode cstore optab/expander,
and only if this fails, falls back to lowering to word mode operations.
On x86_64, this allows the expander to produce:

(set (reg:CCZ 17 flags)
     (compare:CCZ (reg:TI 88)
                  (const_int 0 [0])))

which is a candidate for scalar-to-vector transformations (and
combine simplifications etc.).  On targets that don't define a cstore
pattern for doubleword integer modes, there should be no change in
behaviour.  For those that do, the current behaviour can be restored
(if desired) by restricting the expander/insn to not apply when the
comparison is EQ or NE, and operand[2] is either const0_rtx or
constm1_rtx.

This change just keeps RTL expansion more consistent (in philosophy).
For other doubleword comparisons, such as with operators LT and GT,
or with constants other than zero or -1, the wishes of the backend
are respected, and only if the optab expansion fails are the default
fall-back implementations using narrower integer mode operations
(and conditional jumps) used.

2022-08-05  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* expmed.cc (emit_store_flag_1): Move code to expand double word
	equality and inequality against zero or -1, using word operations,
	to after trying to use the backend's cstore<mode>4 optab/expander.