aarch64: Ignore inductions when costing vector code

aarch64: Ignore inductions when costing vector code

In practice it seems to be better not to cost a vector induction.
The scalar code generally needs the same induction but doesn't
cost it, making an apples-for-apples comparison harder.  Most
inductions also have a low latency and their cost usually gets
hidden by other operations.

Like with the previous patches, this one only becomes active if
a CPU selects use_new_vector_costs.  It should therefore have
a very low impact on other CPUs.

gcc/
	* config/aarch64/aarch64.c (aarch64_detect_vector_stmt_subtype):
	Assume a zero cost for induction phis.