Convert fma, valltrue & vanytrue to ISLE (AArch64) (#4608)

* Convert `fma`, `valltrue` & `vanytrue` to ISLE (AArch64)

Ported the existing implementations of the following opcodes to ISLE on
AArch64:
- `fma`
  - Introduced missing support for `fma` on vector values, as per the
    docs.
- `valltrue`
- `vanytrue`

Also fixed `fcmp` on scalar values in the interpreter, and enabled
interpreter tests in `simd-fma.clif`.

This introduces the `FMLA` machine instruction.

Copyright (c) 2022 Arm Limited

* Add comments for `Fmla` and `Bsl`

Copyright (c) 2022 Arm Limited

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

@@ -292,14 +292,6 @@ impl Imm12 {
         }
     }
 
-    /// Create a zero immediate of this format.
-    pub fn zero() -> Self {
-        Imm12 {
-            bits: 0,
-            shift12: false,
-        }
-    }
-
     /// Bits for 2-bit "shift" field in e.g. AddI.
     pub fn shift_bits(&self) -> u32 {
         if self.shift12 {