[AARCH64][NEON] Allow to sink operands of aarch64_neon_pmull64.

Summary:
This patch fixes a problem when pmull2 instruction is not
generated for vmull_high_p64 intrinsic.

ISel has a pattern for int_aarch64_neon_pmull64 intrinsic to generate
PMULL2 instruction. That pattern assumes that extraction operations
are located in the same basic block. We need to sink them
if they are not. Handle operands of int_aarch64_neon_pmull64
into AArch64TargetLowering::shouldSinkOperands.

Reviewed by: efriedma

Differential Revision: https://reviews.llvm.org/D80320