Convert scalar_to_vector to ISLE (AArch64) (#4401)

* Convert `scalar_to_vector` to ISLE (AArch64)

Converted the exisiting implementation of `scalar_to_vector` for AArch64 to
ISLE.

Copyright (c) 2022 Arm Limited

* Add support for floats and fix FpuExtend

- Added rules to cover `f32 -> f32x4` and `f64 -> f64x2` for
`scalar_to_vector`
- Added tests for `scalar_to_vector` on floats.
- Corrected an invalid instruction emitted by `FpuExtend` on 64-bit
values.

Copyright (c) 2022 Arm Limited

cranelift/codegen/src/isa/aarch64/inst/emit_tests.rs

@@ -5528,6 +5528,16 @@ fn test_aarch64_binemit() {
         "fmov s31, s0",
     ));
 
+    insns.push((
+        Inst::FpuExtend {
+            rd: writable_vreg(31),
+            rn: vreg(0),
+            size: ScalarSize::Size64,
+        },
+        "1F40601E",
+        "fmov d31, d0",
+    ));
+
     insns.push((
         Inst::FpuRR {
             fpu_op: FPUOp1::Abs,