i386: Fix <sse2p4_1>_pinsr<ssemodesuffix> and its splitters [PR103772]

The clever trick to duplicate the value of the input operand into itself
proved not so clever after all.  The splitter should not clobber the input
operand in any case, since the register can hold the value outside the HImode
lowpart when accessed as subreg.  Use the standard earlyclobber approach
instead.

The testcase fails with avx2 ISA, but I was not able to create the testcase
that wouldn't require -mavx512fp16 compile flag.

2021-12-20  Uroš Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog:

	PR target/103772
	* config/i386/sse.md (<sse2p4_1>_pinsr<ssemodesuffix>): Add
	earlyclobber to (x,x,x,i) alternative.
	(<sse2p4_1>_pinsr<ssemodesuffix> peephole2): Remove.
	(<sse2p4_1>_pinsr<ssemodesuffix> splitter): Use output
	operand as a temporary register.  Split after reload_completed.