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/lower/isle.rs

@@ -106,6 +106,16 @@ where
         ImmShift::maybe_from_u64(n.into()).unwrap()
     }
 
+    fn lshr_from_u64(&mut self, ty: Type, n: u64) -> Option<ShiftOpAndAmt> {
+        let shiftimm = ShiftOpShiftImm::maybe_from_shift(n)?;
+        if let Ok(bits) = u8::try_from(ty_bits(ty)) {
+            let shiftimm = shiftimm.mask(bits);
+            Some(ShiftOpAndAmt::new(ShiftOp::LSR, shiftimm))
+        } else {
+            None
+        }
+    }
+
     fn lshl_from_imm64(&mut self, ty: Type, n: Imm64) -> Option<ShiftOpAndAmt> {
         let shiftimm = ShiftOpShiftImm::maybe_from_shift(n.bits() as u64)?;
         let shiftee_bits = ty_bits(ty);