x64 backend: fix cmpxchg (don't return RealReg as result). (#4243)

The current lowering helper for `cmpxchg` returns the literal RealReg
`rax` as its result. However, this breaks a number of invariants, and
eventually causes a regalloc panic if used as a blockparam arg (pinned
vregs cannot be used in this way).

In general we have to return regular vregs, not a RealReg, as results of
instructions during lowering. However #4223 added a helper for
`x64_cmpxchg` that returns a literal `rax`.

Fortunately we can do the right thing here by just giving a fresh vreg
to the instruction; the regalloc constraints mean that this vreg is
constrained to `rax` at the instruction (at its def/late point), so the
generator of the instruction need not worry about `rax` here.

cranelift/codegen/src/isa/x64/lower/isle.rs

@@ -547,12 +547,6 @@ where
     fn zero_offset(&mut self) -> Offset32 {
         Offset32::new(0)
     }
-
-    #[inline]
-    fn rax(&mut self) -> WritableGpr {
-        let gpr = Gpr::new(regs::rax()).unwrap();
-        WritableGpr::from_reg(gpr)
-    }
 }
 
 // Since x64 doesn't have 8x16 shifts and we must use a 16x8 shift instead, we