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/filetests/filetests/runtests/simd-scalartovector-aarch64.clif

@@ -0,0 +1,19 @@
+test run
+target aarch64
+; i8 and i16 are invalid source sizes for x86_64
+
+function %scalartovector_i8(i8) -> i8x16 {
+block0(v0: i8):
+    v1 = scalar_to_vector.i8x16 v0
+    return v1
+}
+; run: %scalartovector_i8(1) == [1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
+; run: %scalartovector_i8(255) == [255 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]
+
+function %scalartovector_i16(i16) -> i16x8 {
+block0(v0: i16):
+    v1 = scalar_to_vector.i16x8 v0
+    return v1
+}
+; run: %scalartovector_i16(1) == [1 0 0 0 0 0 0 0]
+; run: %scalartovector_i16(65535) == [65535 0 0 0 0 0 0 0]