x64: Lower shuffle and swizzle in ISLE (#4772)

Lower `shuffle` and `swizzle` in ISLE.

This PR surfaced a bug with the lowering of `shuffle` when avx512vl and avx512vbmi are enabled: we use `vpermi2b` as the implementation, but panic if the immediate shuffle mask contains any out-of-bounds values. The behavior when the avx512 extensions are not present is that out-of-bounds values are turned into `0` in the result.

I've resolved this by detecting when the shuffle immediate has out-of-bounds indices in the avx512-enabled lowering, and generating an additional mask to zero out the lanes where those indices occur. This brings the avx512 case into line with the semantics of the `shuffle` op: 94bcbe8446/cranelift/codegen/meta/src/shared/instructions.rs (L1495-L1498)

cranelift/codegen/src/machinst/isle.rs

@@ -7,6 +7,7 @@ use std::cell::Cell;
 use target_lexicon::Triple;
 
 pub use super::MachLabel;
+pub use crate::data_value::DataValue;
 pub use crate::ir::{
     ArgumentExtension, Constant, DynamicStackSlot, ExternalName, FuncRef, GlobalValue, Immediate,
     SigRef, StackSlot,
@@ -24,6 +25,7 @@ pub type ValueArray2 = [Value; 2];
 pub type ValueArray3 = [Value; 3];
 pub type WritableReg = Writable<Reg>;
 pub type VecReg = Vec<Reg>;
+pub type VecMask = Vec<u8>;
 pub type ValueRegs = crate::machinst::ValueRegs<Reg>;
 pub type WritableValueRegs = crate::machinst::ValueRegs<WritableReg>;
 pub type InstOutput = SmallVec<[ValueRegs; 2]>;
@@ -683,6 +685,16 @@ macro_rules! isle_prelude_methods {
             Some(u128::from_le_bytes(bytes.try_into().ok()?))
         }
 
+        #[inline]
+        fn vec_mask_from_immediate(&mut self, imm: Immediate) -> Option<VecMask> {
+            let data = self.lower_ctx.get_immediate_data(imm);
+            if data.len() == 16 {
+                Some(Vec::from(data.as_slice()))
+            } else {
+                None
+            }
+        }
+
         #[inline]
         fn u64_from_constant(&mut self, constant: Constant) -> Option<u64> {
             let bytes = self.lower_ctx.get_constant_data(constant).as_slice();