Cranelift AArch64: Further vector constant improvements

Introduce support for MOVI/MVNI with 16-, 32-, and 64-bit elements,
and the vector variant of FMOV.

Copyright (c) 2020, Arm Limited.

cranelift/codegen/src/isa/aarch64/lower.rs

@@ -853,7 +853,7 @@ pub(crate) fn lower_constant_f128<C: LowerCtx<I = Inst>>(
         // is potentially expensive.
         ctx.emit(Inst::VecDupImm {
             rd,
-            imm: ASIMDMovModImm::zero(),
+            imm: ASIMDMovModImm::zero(ScalarSize::Size8),
             invert: false,
             size: VectorSize::Size8x16,
         });