testsuite: arm: bf16_vstn_1 vst3q_bf16 improved by late-combine

The late-combine pass removes some unnecessary register copying in
bf16_vstn_1.c, copying that was expected by vst3q_bf16.  Adjust the
expectations so that they match the better code we get now.

for  gcc/testsuite/ChangeLog

	* gcc.target/arm/simd/bf16_vstn_1.c: Adjust expectations for
	code improved by late-combine.

(cherry picked from commit b4f09e9f8fd7e9fa8e132a03b510a3aaca78260f)