i386: Fix up cvtsd2ss splitter [PR104502]

The following testcase ICEs, because AVX512F is enabled, AVX512VL is not,
and the cvtsd2ss insn has %xmm0-15 as output operand and %xmm16-31 as
input operand.  For output operand %xmm16+ the splitter just gives up
in such case, but for such input it just emits vmovddup which requires
AVX512VL if either operand is EXT_REX_SSE_REG_P (when it is 128-bit).

The following patch fixes it by treating that case like the pre-SSE3
output != input case - move the input to output and do everything on
the output reg which is known to be < %xmm16.

2022-02-12  Jakub Jelinek  <jakub@redhat.com>

	PR target/104502
	* config/i386/i386.md (cvtsd2ss splitter): If operands[1] is xmm16+
	and AVX512VL isn't available, move operands[1] to operands[0] first.

	* gcc.target/i386/pr104502.c: New test.