[AArch64] Change the type of i64 neon shifts to v1i64

This alters the lowering of shifts by a constant, so that the type is lowered
to a v1i64 instead of a i64. This helps communicate that the type will live in
a neon register, and can help clean up surrounding code. Note this is only
currently for the scalar shifts of a constant that go through the nodes in
tryCombineShiftImm.

ssra instructions are no longer being recognized in places, but that can be
cleaned up in a followup patch that combines the i64 add into a v1i64 add.

Differential Revision: https://reviews.llvm.org/D148309