x64: Add support for the pblendw instruction (#6023)

This commit adds another case for `shuffle` lowering to the x64 backend
for the `{,v}pblendw` instruction. This instruction selects 16-bit
values from either of the inputs corresponding to an immediate 8-bit-mask where
each bit selects the corresponding lane from the inputs.

cranelift/codegen/src/isa/x64/inst/emit.rs

@@ -2263,6 +2263,7 @@ pub(crate) fn emit(
                 AvxOpcode::Vpalignr => (false, LegacyPrefixes::_66, OpcodeMap::_0F3A, 0x0F),
                 AvxOpcode::Vinsertps => (false, LegacyPrefixes::_66, OpcodeMap::_0F3A, 0x21),
                 AvxOpcode::Vshufps => (false, LegacyPrefixes::None, OpcodeMap::_0F, 0xC6),
+                AvxOpcode::Vpblendw => (false, LegacyPrefixes::_66, OpcodeMap::_0F3A, 0x0E),
                 _ => panic!("unexpected rmr_imm_vex opcode {op:?}"),
             };
 
@@ -2719,6 +2720,7 @@ pub(crate) fn emit(
                 SseOpcode::Pinsrw => (LegacyPrefixes::_66, 0x0FC4, 2),
                 SseOpcode::Pinsrd => (LegacyPrefixes::_66, 0x0F3A22, 3),
                 SseOpcode::Shufps => (LegacyPrefixes::None, 0x0FC6, 2),
+                SseOpcode::Pblendw => (LegacyPrefixes::_66, 0x0F3A0E, 3),
                 _ => unimplemented!("Opcode {:?} not implemented", op),
             };
             let rex = RexFlags::from(*size);