machinst x64: use a sign-extension when loading jump table offsets;

The jump table offset that's loaded out of the jump table could be
signed (if it's an offset to before the jump table itself), so we should
use a signed extension there, not an unsigned extension.

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

@@ -350,6 +350,7 @@ pub enum Inst {
     /// Jump-table sequence, as one compound instruction (see note in lower.rs for rationale).
     /// The generated code sequence is described in the emit's function match arm for this
     /// instruction.
+    /// See comment in lowering about the temporaries signedness.
     JmpTableSeq {
         idx: Reg,
         tmp1: Writable<Reg>,