varasm: Reject soft frame or arg pointer registers for register vars [PR92469]

varasm: Reject soft frame or arg pointer registers for register vars [PR92469]

The following patch rejects frame, argp and retarg registers (unless they are equal
to hard frame pointer registers or if they aren't eliminable) from local or global
register vars.
These are just internal implementation details eliminated later into hard
frame pointer or stack pointer and using them as register variable leads
to numerous ICEs.

2020-12-13  Jakub Jelinek  <jakub@redhat.com>

	PR target/92469
	* varasm.c (eliminable_regno_p): New function.
	(make_decl_rtl): Reject asm vars for frame and argp
	if they are different from hard frame pointer.

	* gcc.target/i386/pr92469.c: New test.
	* gcc.target/i386/pr79804.c: Adjust expected diagnostics.
	* gcc.target/i386/pr88178.c: Expect an error.