RTL: Add `const_double_zero' syntactic rtx

RTL: Add `const_double_zero' syntactic rtx

The use of a constant double zero is required for post-reload compare
elimination to be able to discard redundant floating-point comparisons,
for example with a VAX RTL instruction stream like:

(insn 34 4 3 2 (parallel [
            (set (reg/v:DF 0 %r0 [orig:24 x ] [24])
                (mem/c:DF (plus:SI (reg/f:SI 12 %ap)
                        (const_int 4 [0x4])) [1 x+0 S8 A32]))
            (clobber (reg:CC 16 %psl))
        ]) ".../gcc/testsuite/gcc.target/vax/cmpelim-eq-movdf.c":9:1 37 {*movdf}
     (nil))
(note 3 34 35 2 NOTE_INSN_FUNCTION_BEG)
(insn 35 3 36 2 (set (reg:CCZ 16 %psl)
        (compare:CCZ (reg/v:DF 0 %r0 [orig:24 x ] [24])
            (const_double:DF 0.0 [0x0.0p+0]))) ".../gcc/testsuite/gcc.target/vax/cmpelim-eq-movdf.c":10:6 21 {*cmpdf_ccz}
     (nil))
(jump_insn 36 35 9 2 (set (pc)
        (if_then_else (eq (reg:CCZ 16 %psl)
                (const_int 0 [0]))
            (label_ref 11)
            (pc))) ".../gcc/testsuite/gcc.target/vax/cmpelim-eq-movdf.c":10:6 537 {*branch_ccz}
     (int_list:REG_BR_PROB 536870916 (nil))
 -> 11)

that we want to transform into:

(insn 34 4 3 2 (parallel [
            (set (reg:CCZ 16 %psl)
                (compare:CCZ (mem/c:DF (plus:SI (reg/f:SI 12 %ap)
                            (const_int 4 [0x4])) [1 x+0 S8 A32])
                    (const_double:DF 0.0 [0x0.0p+0])))
            (set (reg/v:DF 0 %r0 [orig:24 x ] [24])
                (mem/c:DF (plus:SI (reg/f:SI 12 %ap)
                        (const_int 4 [0x4])) [1 x+0 S8 A32]))
        ]) ".../gcc/testsuite/gcc.target/vax/cmpelim-eq-movdf.c":9:1 40 {*movdf_ccz}
     (nil))
(note 3 34 36 2 NOTE_INSN_FUNCTION_BEG)
(jump_insn 36 3 9 2 (set (pc)
        (if_then_else (eq (reg:CCZ 16 %psl)
                (const_int 0 [0]))
            (label_ref 11)
            (pc))) ".../gcc/testsuite/gcc.target/vax/cmpelim-eq-movdf.c":10:6 537 {*branch_ccz}
     (int_list:REG_BR_PROB 536870916 (nil))
 -> 11)

with the upcoming MODE_CC representation.

For this we need to express the `const_double:DF 0.0 [0x0.0p+0]' rtx as
recorded above in the relevant pattern(s) in machine description.  The
way we represent double constants, as a host-dependent number of wide
integers, however means that we currently have no portable way to encode
a double zero constant in machine description.

Define a syntactic rtx alias then to represent `(const_double 0 0 ...)'
as if the suitable number of zeros have been supplied according to the
host-specific definition of CONST_DOUBLE_FORMAT.

	gcc/
	* read-rtl.c (rtx_reader::read_rtx_code): Handle syntactic
	`const_double_zero' rtx.
	* doc/rtl.texi (Constant Expression Types): Document it.