aarch64: Restrict issue heuristics to inner vector loop

aarch64: Restrict issue heuristics to inner vector loop

The AArch64 vector costs try to take issue rates into account.
However, when vectorising an outer loop, we lumped the inner
and outer operations together, which is somewhat meaningless.
This patch restricts the heuristic to the inner loop.

gcc/
	* config/aarch64/aarch64.c (aarch64_add_stmt_cost): Only
	record issue information for operations that occur in the
	innermost loop.