rs6000: Use rldimi for 64-bit constants with high=low (PR93012)

rs6000: Use rldimi for 64-bit constants with high=low (PR93012)

We currently use an (up to) five instruction sequence to generate such
constants.  After this change we just generate a 32-bit constant and do
a rotate-and-mask-insert instruction, making the sequence only up to
three instructions.

	* config/rs6000/rs6000.c (rs6000_emit_set_long_const): Handle the case
	where the low and the high 32 bits are equal to each other specially,
	with an rldimi instruction.

gcc/testsuite/
	* gcc.target/powerpc/pr93012.c: New.