x64: handle tests of b1 values correctly (only LSB is defined).

Previously, `select` and `brz`/`brnz` instructions, when given a `b1`
boolean argument, would test whether that boolean argument was nonzero,
rather than whether its LSB was nonzero. Since our invariant for mapping
CLIF state to machine state is that bits beyond the width of a value are
undefined, the proper lowering is to test only the LSB.

(aarch64 does not have the same issue because its `Extend` pseudoinst
already properly handles masking of b1 values when a zero-extend is
requested, as it is for select/brz/brnz.)

Found by Nathan Ringo on Zulip [1] (thanks!).

[1]
https://bytecodealliance.zulipchat.com/#narrow/stream/217117-cranelift/topic/bnot.20on.20b1s

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

@@ -397,6 +397,14 @@ impl fmt::Display for UnaryRmROpcode {
     }
 }
 
+#[derive(Clone, Copy, PartialEq)]
+pub enum CmpOpcode {
+    /// CMP instruction: compute `a - b` and set flags from result.
+    Cmp,
+    /// TEST instruction: compute `a & b` and set flags from result.
+    Test,
+}
+
 pub(crate) enum InstructionSet {
     SSE,
     SSE2,