[AArch64] Add -fno-vect-cost-model to sve2/whilerw_1.c

[AArch64] Add -fno-vect-cost-model to sve2/whilerw_1.c

Bumping the cost of vec_to_scalar made the .s loop in
gcc.target/aarch64/sve2/whilerw_1.c use a runtime profitability check,
like the .d version already did.  Since the cost model isn't really
being tested here, the most robust fix seemed to be to disable it,
which I should really have done from the outset.

2019-11-21  Richard Sandiford  <richard.sandiford@arm.com>

gcc/testsuite/
	* gcc.target/aarch64/sve2/whilerw_1.c: Add -fno-vect-cost-model.
	Require x0 in the .d test too.

From-SVN: r278549