Add missing natual vector cast.

Summary: The natual vector cast node (similar to bitcast) AArch64ISD::NVCAST
was introduced in r217159 and r217138. This patch adds a missing cast from
v2f32 to v1i64 which is causing some compilation failures. Also added test
cases to cover various modimm types and BUILD_VECTORs with i64 elements.

llvm-svn: 218751