[X86][BtVer2] Fix latency and resource cycles of AVX 256-bit zero-idioms.

This patch introduces a SchedWriteVariant to describe zero-idiom VXORP(S|D)Yrr
and VANDNP(S|D)Yrr.

This is a follow-up of r342555.

On Jaguar, a VXORPSYrr is 2 macro opcodes. Only one opcode is eliminated at
register-renaming stage. The other opcode has to be executed to set the upper
half of the destination YMM.
Same for VANDNP(S|D)Yrr.

Differential Revision: https://reviews.llvm.org/D52347

llvm-svn: 342728