aarch64: Improve size heuristic for cpymem expansion

aarch64: Improve size heuristic for cpymem expansion

Similar to my previous patch for setmem this one does the same for the cpymem expansion.
We count the number of ops emitted and compare it against the alternative of just calling
the library function when optimising for size.
For the code:
void
cpy_127 (char *out, char *in)
{
  __builtin_memcpy (out, in, 127);
}

void
cpy_128 (char *out, char *in)
{
  __builtin_memcpy (out, in, 128);
}

we now emit a call to memcpy (with an extra MOV-immediate instruction for the size) instead of:
cpy_127(char*, char*):
        ldp     q0, q1, [x1]
        stp     q0, q1, [x0]
        ldp     q0, q1, [x1, 32]
        stp     q0, q1, [x0, 32]
        ldp     q0, q1, [x1, 64]
        stp     q0, q1, [x0, 64]
        ldr     q0, [x1, 96]
        str     q0, [x0, 96]
        ldr     q0, [x1, 111]
        str     q0, [x0, 111]
        ret
cpy_128(char*, char*):
        ldp     q0, q1, [x1]
        stp     q0, q1, [x0]
        ldp     q0, q1, [x1, 32]
        stp     q0, q1, [x0, 32]
        ldp     q0, q1, [x1, 64]
        stp     q0, q1, [x0, 64]
        ldp     q0, q1, [x1, 96]
        stp     q0, q1, [x0, 96]
        ret

which is a clear code size win. Speed optimisation heuristics remain unchanged.

2021-09-29  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

	* config/aarch64/aarch64.c (aarch64_expand_cpymem): Count number of
	emitted operations and adjust heuristic for code size.

2021-09-29  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

	* gcc.target/aarch64/cpymem-size.c: New test.