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.isle

@@ -1637,6 +1637,13 @@
             (_ Unit (emit (MInst.Extend dst rn signed from_bits to_bits))))
         dst))
 
+;; Helper for emitting `MInst.FpuExtend` instructions.
+(decl fpu_extend (Reg ScalarSize) Reg)
+(rule (fpu_extend src size)
+      (let ((dst WritableReg (temp_writable_reg $F32X4))
+            (_ Unit (emit (MInst.FpuExtend dst src size))))
+        dst))
+
 ;; Helper for emitting `MInst.LoadAcquire` instructions.
 (decl load_acquire (Type Reg) Reg)
 (rule (load_acquire ty addr)