i386: Rewrite bswaphi2 handling [PR115102]

i386: Rewrite bswaphi2 handling [PR115102]

Introduce *bswaphi2 instruction pattern and enable bswaphi2 expander
also for non-movbe targets.  The testcase:

unsigned short bswap8 (unsigned short val)
{
  return ((val & 0xff00) >> 8) | ((val & 0xff) << 8);
}

now expands through bswaphi2 named expander.

Rewrite bswaphi_lowpart insn pattern as bswaphisi2_lowpart in the RTX form
that combine pass can use to simplify:

Trying 6, 9, 8 -> 10:
    6: r99:SI=bswap(r103:SI)
    9: {r107:SI=r103:SI&0xffffffffffff0000;clobber flags:CC;}
      REG_DEAD r103:SI
      REG_UNUSED flags:CC
    8: {r106:SI=r99:SI 0>>0x10;clobber flags:CC;}
      REG_DEAD r99:SI
      REG_UNUSED flags:CC
   10: {r104:SI=r106:SI|r107:SI;clobber flags:CC;}
      REG_DEAD r107:SI
      REG_DEAD r106:SI
      REG_UNUSED flags:CC

Successfully matched this instruction:
(set (reg:SI 104 [ _8 ])
    (ior:SI (and:SI (reg/v:SI 103 [ val ])
            (const_int -65536 [0xffffffffffff0000]))
        (lshiftrt:SI (bswap:SI (reg/v:SI 103 [ val ]))
            (const_int 16 [0x10]))))
allowing combination of insns 6, 8, 9 and 10

when compiling the following testcase:

unsigned int bswap8 (unsigned int val)
{
  return (val & 0xffff0000) | ((val & 0xff00) >> 8) | ((val & 0xff) << 8);
}

to produce:

	movl    %edi, %eax
	xchgb   %ah, %al
	ret

The expansion now always goes through a clobberless form of the bswaphi
instruction.  The instruction is conditionally converted to a rotate at
peephole2 pass.  This significantly simplifies bswaphisi2_lowpart
insn pattern attributes.

	PR target/115102

gcc/ChangeLog:

	* config/i386/i386.md (bswaphi2): Also enable for !TARGET_MOVBE.
	(*bswaphi2): New insn pattern.
	(bswaphisi2_lowpart): Rename from bswaphi_lowpart.  Rewrite
	insn RTX to match the expected form of the combine pass.
	Remove rol{w} alternative and corresponding attributes.
	(bswsaphisi2_lowpart peephole2): New peephole2 pattern to
	conditionally convert bswaphisi2_lowpart to rotlhi3_1_slp.
	(bswapsi2): Update expander for rename.
	(rotlhi3_1_slp splitter): Conditionally split to bswaphi2.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr115102.c: New test.