IBM Z: Use llihf and oilf to load large immediates into GPRs

IBM Z: Use llihf and oilf to load large immediates into GPRs

Currently GCC loads large immediates into GPRs from the literal pool,
which is not as efficient as loading two halves with llihf and oilf.

gcc/ChangeLog:

2020-11-30  Ilya Leoshkevich  <iii@linux.ibm.com>

	* config/s390/s390-protos.h (s390_const_int_pool_entry_p): New
	function.
	* config/s390/s390.c (s390_const_int_pool_entry_p): New
	function.
	* config/s390/s390.md: Add define_peephole2 that produces llihf
	and oilf.

gcc/testsuite/ChangeLog:

2020-11-30  Ilya Leoshkevich  <iii@linux.ibm.com>

	* gcc.target/s390/load-imm64-1.c: New test.
	* gcc.target/s390/load-imm64-2.c: New test.