LoongArch: Don't split the instructions containing relocs for extreme code model.

LoongArch: Don't split the instructions containing relocs for extreme code model.

The ABI mandates the pcalau12i/addi.d/lu32i.d/lu52i.d instructions for
addressing a symbol to be adjacent.  So model them as "one large
instruction", i.e. define_insn, with two output registers.  The real
address is the sum of these two registers.

The advantage of this approach is the RTL passes can still use ldx/stx
instructions to skip an addi.d instruction.

gcc/ChangeLog:

	* config/loongarch/loongarch.md (unspec): Add
	UNSPEC_LA_PCREL_64_PART1 and UNSPEC_LA_PCREL_64_PART2.
	(la_pcrel64_two_parts): New define_insn.
	* config/loongarch/loongarch.cc (loongarch_tls_symbol): Fix a
	typo in the comment.
	(loongarch_call_tls_get_addr): If -mcmodel=extreme
	-mexplicit-relocs={always,auto}, use la_pcrel64_two_parts for
	addressing the TLS symbol and __tls_get_addr.  Emit an REG_EQUAL
	note to allow CSE addressing __tls_get_addr.
	(loongarch_legitimize_tls_address): If -mcmodel=extreme
	-mexplicit-relocs={always,auto}, address TLS IE symbols with
	la_pcrel64_two_parts.
	(loongarch_split_symbol): If -mcmodel=extreme
	-mexplicit-relocs={always,auto}, address symbols with
	la_pcrel64_two_parts.
	(loongarch_output_mi_thunk): Clean up unreachable code.  If
	-mcmodel=extreme -mexplicit-relocs={always,auto}, address the MI
	thunks with la_pcrel64_two_parts.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/func-call-extreme-1.c (dg-options):
	Use -O2 instead of -O0 to ensure the pcalau12i/addi/lu32i/lu52i
	instruction sequences are not reordered by the compiler.
	(NOIPA): Disallow interprocedural optimizations.
	* gcc.target/loongarch/func-call-extreme-2.c: Remove the content
	duplicated from func-call-extreme-1.c, include it instead.
	(dg-options): Likewise.
	* gcc.target/loongarch/func-call-extreme-3.c (dg-options):
	Likewise.
	* gcc.target/loongarch/func-call-extreme-4.c (dg-options):
	Likewise.
	* gcc.target/loongarch/cmodel-extreme-1.c: New test.
	* gcc.target/loongarch/cmodel-extreme-2.c: New test.
	* g++.target/loongarch/cmodel-extreme-mi-thunk-1.C: New test.
	* g++.target/loongarch/cmodel-extreme-mi-thunk-2.C: New test.
	* g++.target/loongarch/cmodel-extreme-mi-thunk-3.C: New test.