Implement __imag__ of float _Complex using shufps on x86_64.

This patch is a follow-up improvement to my recent patch for
PR rtl-optimization/7061.  That patch added the test case
gcc.target/i386/pr7061-2.c:

float im(float _Complex a) { return __imag__ a; }

For which GCC on x86_64 currently generates:

        movq    %xmm0, %rax
        shrq    $32, %rax
        movd    %eax, %xmm0
        ret

but with this patch we now generate (the same as LLVM):

        shufps  $85, %xmm0, %xmm0
        ret

This is achieved by providing a define_insn_and_split that allows
truncated lshiftrt:DI by 32 to be performed on either SSE or general
regs, where if the register allocator prefers to use SSE, we split
to a shufps_v4si, or if not, we use a regular shrq.

2022-06-27  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR rtl-optimization/7061
	* config/i386/i386.md (*highpartdisi2): New define_insn_and_split.

gcc/testsuite/ChangeLog
	PR rtl-optimization/7061
	* gcc.target/i386/pr7061-2.c: Update to look for shufps.

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