Prevent clobbering of TAILCALL op2 in epilogue

[arnaud-lb]

TAILCALL op2 may be overridden when it is assigned to a preserved register, as the epilogue is emitted before the call:

{
	l_1 = START(l_4);
	uintptr_t f = PARAM(l_1, "p", 1);
	l_2 = CALL(l_1, f);
	l_3 = TAILCALL(l_2, f);
	l_4 = UNREACHABLE(l_3);
}

test:
	pushq %rbx
	movq %rdi, %rbx
	callq *%rbx
	popq %rbx
	jmpq *%rbx

This can not be trivially solved in RA, as the set of used preserved regs is not known yet.

Here I move op2 to a tmp reg in ir_emit_tailcall() when op2 is in used_preserved_regs.

[dstogov]

Thanks for the attempt to fix this and for the new test cases.
I think, the idea should work, but I see few problems.
See comments...

We probably should also check for %rsp and %rbp in the fused memory load.

[dstogov]

Can we have a spilled op2_reg that not interference with ctx->used_preserved_reg?
I think you lost this case.

[arnaud-lb]

Yes, I missed this case. I've updated the PR and added tests for that.

[dstogov]

Use ir_emit_load_mem_int()

[dstogov]

The code looks wrong. %rsp is changed between movq %rbx, (%rsp) and jmpq *(%rsp)

[arnaud-lb]

Indeed, I didn't notice that