Optimize signed DImode -> TImode on power10.

Optimize signed DImode -> TImode on power10.

On power10, GCC tries to optimize the signed conversion from DImode to
TImode by using the vextsd2q instruction.  However to generate this
instruction, it would have to generate 3 direct moves (1 from the GPR
registers to the altivec registers, and 2 from the altivec registers to
the GPR register).

This patch generates the shift right immediate instruction to do the
conversion if the target/source registers ares GPR registers like it does
on earlier systems.  If the target/source registers are Altivec registers,
it will generate the vextsd2q instruction.

2022-03-05   Michael Meissner  <meissner@linux.ibm.com>

gcc/
	PR target/104698
	* config/rs6000/vsx.md (UNSPEC_MTVSRD_DITI_W1): Delete.
	(mtvsrdd_diti_w1): Delete.
	(extendditi2): Convert from define_expand to
	define_insn_and_split.  Replace with code to deal with both GPR
	registers and with altivec registers.

gcc/testsuite/
	PR target/104698
	* gcc.target/powerpc/pr104698-1.c: New test.
	* gcc.target/powerpc/pr104698-2.c: New test.