RTL: Update `const_double_zero' handling for mode and callable insns

RTL: Update `const_double_zero' handling for mode and callable insns

Handle machine mode specification with `const_double_zero' and handle
the rtx with callable code produced from named insns.  Complementing
commit 20ab43b5cad6 ("RTL: Add `const_double_zero' syntactic rtx") and
removing a commit c60d0736dff7 ("PDP11: Use `const_double_zero' to
express double zero constant") build regression observed with the
`pdp11-aout' target:

genemit: Internal error: abort in gen_exp, at genemit.c:202
make[2]: *** [Makefile:2427: s-emit] Error 1

where a:

(const_double 0 [0] 0 [0] 0 [0] 0 [0])

rtx coming from:

(parallel [
        (set (reg:CC 16)
            (compare:CC (abs:DF (match_operand:DF 1 ("general_operand") ("0,0")))
                (const_double 0 [0] 0 [0] 0 [0] 0 [0])))
        (set (match_operand:DF 0 ("nonimmediate_operand") ("=fR,Q"))
            (abs:DF (match_dup 1)))
    ])

and ultimately `(const_double_zero)' referred in a named RTL insn cannot
be interpreted.  Handle the rtx then by supplying the constant 0 double
operand requested, resulting in the following update to insn-emit.c code
produced for the `pdp11-aout' target, relative to before the triggering
commit:

@@ -1514,7 +1514,7 @@ gen_absdf2_cc (rtx operand0 ATTRIBUTE_UN
 	gen_rtx_COMPARE (CCmode,
 	gen_rtx_ABS (DFmode,
 	operand1),
-	const0_rtx)),
+	CONST_DOUBLE_ATOF ("0", VOIDmode))),
 		gen_rtx_SET (operand0,
 	gen_rtx_ABS (DFmode,
 	copy_rtx (operand1)))));
@@ -1555,7 +1555,7 @@ gen_negdf2_cc (rtx operand0 ATTRIBUTE_UN
 	gen_rtx_COMPARE (CCmode,
 	gen_rtx_NEG (DFmode,
 	operand1),
-	const0_rtx)),
+	CONST_DOUBLE_ATOF ("0", VOIDmode))),
 		gen_rtx_SET (operand0,
 	gen_rtx_NEG (DFmode,
 	copy_rtx (operand1)))));
@@ -1790,7 +1790,7 @@ gen_muldf3_cc (rtx operand0 ATTRIBUTE_UN
 	gen_rtx_MULT (DFmode,
 	operand1,
 	operand2),
-	const0_rtx)),
+	CONST_DOUBLE_ATOF ("0", VOIDmode))),
 		gen_rtx_SET (operand0,
 	gen_rtx_MULT (DFmode,
 	copy_rtx (operand1),
@@ -1942,7 +1942,7 @@ gen_divdf3_cc (rtx operand0 ATTRIBUTE_UN
 	gen_rtx_DIV (DFmode,
 	operand1,
 	operand2),
-	const0_rtx)),
+	CONST_DOUBLE_ATOF ("0", VOIDmode))),
 		gen_rtx_SET (operand0,
 	gen_rtx_DIV (DFmode,
 	copy_rtx (operand1),

This does not (yet) remove VOIDmode CONST_DOUBLE use, as it is up to
individual machine descriptions to choose.

	gcc/
	* genemit.c (gen_exp) <CONST_DOUBLE>: Handle `const_double_zero'
	rtx.
	* read-rtl.c (rtx_reader::read_rtx_code): Handle machine mode
	with `const_double_zero'.
	* doc/rtl.texi (Constant Expression Types): Document it.