Provide zero_extend versions/variants of several patterns on x86.

Provide zero_extend versions/variants of several patterns on x86.

The middle-end doesn't have a preferred canonical form for expressing
zero-extension, sometimes using an AND, sometimes pairs of SHIFTs,
and sometimes using zero_extend.  Pending changes to RTL simplification
will/may alter some of these representations, so a few additional
patterns are required to recognize these alternate representations
and avoid any testsuite regressions.

As an example, *popcountsi2_zext is currently represented as:
  [(set (match_operand:DI 0 "register_operand" "=r")
        (and:DI
          (subreg:DI
            (popcount:SI
              (match_operand:SI 1 "nonimmediate_operand" "rm")) 0)
          (const_int 63)))
   (clobber (reg:CC FLAGS_REG))]

this patch adds an alternate/equivalent pattern that matches:
  [(set (match_operand:DI 0 "register_operand" "=r")
       (zero_extend:DI
         (popcount:SI (match_operand:SI 1 "nonimmediate_operand" "rm"))))
   (clobber (reg:CC FLAGS_REG))]

Another example is *popcounthi2 which is currently represented as:
  [(set (match_operand:SI 0 "register_operand")
        (popcount:SI
          (zero_extend:SI (match_operand:HI 1 "nonimmediate_operand"))))
   (clobber (reg:CC FLAGS_REG))]

this patch adds an alternate/equivalent pattern that matches:
  [(set (match_operand:SI 0 "register_operand")
        (zero_extend:SI
          (popcount:HI (match_operand:HI 1 "nonimmediate_operand"))))
   (clobber (reg:CC FLAGS_REG))]

The contents of the machine description definitions remain the same.
it's just the expected RTL is slightly different but equivalent.
Providing both forms makes the backend more robust to middle-end
changes [and possibly catches some missed optimizations].

2022-12-28  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386.md (*clzsi2_lzcnt_zext_2): define_insn_and_split
	to match ZERO_EXTEND form of *clzsi2_lzcnt_zext.
	(*clzsi2_lzcnt_zext_2_falsedep): Likewise, new define_insn to match
	ZERO_EXTEND form of *clzsi2_lzcnt_zext_falsedep.
	(*bmi2_bzhi_zero_extendsidi_5): Likewise, new define_insn to match
	ZERO_EXTEND form of *bmi2_bzhi_zero_extendsidi.
	(*popcountsi2_zext_2): Likewise, new define_insn_and_split to match
	ZERO_EXTEND form of *popcountsi2_zext.
	(*popcountsi2_zext_2_falsedep): Likewise, new define_insn to match
	ZERO_EXTEND form of *popcountsi2_zext_falsedep.
	(*popcounthi2_2): Likewise, new define_insn_and_split to match
	ZERO_EXTEND form of *popcounthi2.
	(define_peephole2): ZERO_EXTEND variant of HImode popcount&1 using
	parity flag peephole2.