x64: add support for packed promote and demote (#2783)

* Add support for x64 packed promote low

* Add support for x64 packed floating point demote

* Update vector promote low and demote by adding constraints

Also does some renaming and minor refactoring

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

@@ -1348,6 +1348,8 @@ pub(crate) fn emit(
 
             let (prefix, opcode, num_opcodes) = match op {
                 SseOpcode::Cvtdq2pd => (LegacyPrefixes::_F3, 0x0FE6, 2),
+                SseOpcode::Cvtpd2ps => (LegacyPrefixes::_66, 0x0F5A, 2),
+                SseOpcode::Cvtps2pd => (LegacyPrefixes::None, 0x0F5A, 2),
                 SseOpcode::Cvtss2sd => (LegacyPrefixes::_F3, 0x0F5A, 2),
                 SseOpcode::Cvtsd2ss => (LegacyPrefixes::_F2, 0x0F5A, 2),
                 SseOpcode::Movaps => (LegacyPrefixes::None, 0x0F28, 2),