i386: Adjust -fzero-call-used-regs to always use XOR [PR101891]

i386: Adjust -fzero-call-used-regs to always use XOR [PR101891]

Currently on i386, -fzero-call-used-regs uses a pattern of:

XOR regA,regA
MOV regA,regB
MOV regA,regC
...
RET

However, this introduces both a register ordering dependency (e.g. the CPU
cannot clear regB without clearing regA first), and while greatly reduces
available ROP gadgets, it does technically leave a set of "MOV" ROP gadgets
at the end of functions (e.g. "MOV regA,regC; RET").

This patch will switch to always use XOR on i386:

XOR regA,regA
XOR regB,regB
XOR regC,regC
...
RET

gcc/ChangeLog:

	PR target/101891
	* config/i386/i386.cc (zero_call_used_regno_mode): use V2SImode
	as a generic MMX mode instead of V4HImode.
	(zero_all_mm_registers): Use SET to zero instead of MOV for
	zeroing scratch registers.
	(ix86_zero_call_used_regs): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/zero-scratch-regs-1.c: Add -fno-stack-protector
	-fno-PIC.
	* gcc.target/i386/zero-scratch-regs-10.c: Adjust mov to xor.
	* gcc.target/i386/zero-scratch-regs-13.c: Add -msse.
	* gcc.target/i386/zero-scratch-regs-14.c: Adjust mov to xor.
	* gcc.target/i386/zero-scratch-regs-15.c: Add -fno-stack-protector
	-fno-PIC.
	* gcc.target/i386/zero-scratch-regs-16.c: Likewise.
	* gcc.target/i386/zero-scratch-regs-17.c: Likewise.
	* gcc.target/i386/zero-scratch-regs-18.c: Add -fno-stack-protector
	-fno-PIC, adjust mov to xor.
	* gcc.target/i386/zero-scratch-regs-19.c: Add -fno-stack-protector
	-fno-PIC.
	* gcc.target/i386/zero-scratch-regs-2.c: Adjust mov to xor.
	* gcc.target/i386/zero-scratch-regs-20.c: Add -msse.
	* gcc.target/i386/zero-scratch-regs-21.c: Add -fno-stack-protector
	-fno-PIC, Adjust mov to xor.
	* gcc.target/i386/zero-scratch-regs-22.c: Adjust mov to xor.
	* gcc.target/i386/zero-scratch-regs-23.c: Likewise.
	* gcc.target/i386/zero-scratch-regs-26.c: Likewise.
	* gcc.target/i386/zero-scratch-regs-27.c: Likewise.
	* gcc.target/i386/zero-scratch-regs-28.c: Likewise.
	* gcc.target/i386/zero-scratch-regs-3.c: Add -fno-stack-protector.
	* gcc.target/i386/zero-scratch-regs-31.c: Adjust mov to xor.
	* gcc.target/i386/zero-scratch-regs-4.c: Add -fno-stack-protector
	-fno-PIC.
	* gcc.target/i386/zero-scratch-regs-5.c: Adjust mov to xor.
	* gcc.target/i386/zero-scratch-regs-6.c: Add -fno-stack-protector.
	* gcc.target/i386/zero-scratch-regs-7.c: Likewise.
	* gcc.target/i386/zero-scratch-regs-8.c: Adjust mov to xor.
	* gcc.target/i386/zero-scratch-regs-9.c: Add -fno-stack-protector.