aarch64: Don't emit invalid zero/sign-extend syntax

aarch64: Don't emit invalid zero/sign-extend syntax

Given the following C function:

double *f(double *p, unsigned x)
{
    return p + x;
}

prior to this patch, GCC at -O2 would generate:

f:
        add     x0, x0, x1, uxtw 3
        ret

but this add instruction uses architecturally-invalid syntax: the width
of the third operand conflicts with the width of the extension
specifier. The third operand is only permitted to be an x register when
the extension specifier is (u|s)xtx.

This instruction, and analogous insns for adds, sub, subs, and cmp, are
rejected by clang, but accepted by binutils. Assembling and
disassembling such an insn with binutils gives the architecturally-valid
version in the disassembly:

   0:   8b214c00        add     x0, x0, w1, uxtw #3

This patch fixes several patterns in the AArch64 backend to use the
standard syntax as specified in the Arm ARM such that GCC's output can
be assembled by assemblers other than GAS.

---

gcc/ChangeLog:

	* config/aarch64/aarch64.md
	(*adds_<optab><ALLX:mode>_<GPI:mode>): Ensure extended operand
	agrees with width of extension specifier.
	(*subs_<optab><ALLX:mode>_<GPI:mode>): Likewise.
	(*adds_<optab><ALLX:mode>_shift_<GPI:mode>): Likewise.
	(*subs_<optab><ALLX:mode>_shift_<GPI:mode>): Likewise.
	(*add_<optab><ALLX:mode>_<GPI:mode>): Likewise.
	(*add_<optab><ALLX:mode>_shft_<GPI:mode>): Likewise.
	(*add_uxt<mode>_shift2): Likewise.
	(*sub_<optab><ALLX:mode>_<GPI:mode>): Likewise.
	(*sub_<optab><ALLX:mode>_shft_<GPI:mode>): Likewise.
	(*sub_uxt<mode>_shift2): Likewise.
	(*cmp_swp_<optab><ALLX:mode>_reg<GPI:mode>): Likewise.
	(*cmp_swp_<optab><ALLX:mode>_shft_<GPI:mode>): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/adds3.c: Fix test w.r.t. new syntax.
	* gcc.target/aarch64/cmp.c: Likewise.
	* gcc.target/aarch64/subs3.c: Likewise.
	* gcc.target/aarch64/subsp.c: Likewise.
	* gcc.target/aarch64/extend-syntax.c: New test.