Revert "Add fixed-non-allocatable operand support."

This feature needs more thought; for now we will of course continue to
support pinned vregs, but perhaps we can do better for
"pass-through-and-forget" operands that are given non-allocatable
registers.

This reverts commit 736f636c36.

src/lib.rs

@@ -144,15 +144,6 @@ impl VReg {
     pub fn invalid() -> Self {
         VReg::new(Self::MAX, RegClass::Int)
     }
-
-    #[inline(always)]
-    pub fn is_valid(self) -> bool {
-        self != Self::invalid()
-    }
-    #[inline(always)]
-    pub fn is_invalid(self) -> bool {
-        self == Self::invalid()
-    }
 }
 
 impl std::fmt::Debug for VReg {
@@ -337,19 +328,6 @@ impl Operand {
         )
     }
 
-    /// Create an Operand that always results in an assignment to the
-    /// given fixed `preg`, *without* tracking liveranges in that
-    /// `preg`. Must only be used for non-allocatable registers.
-    #[inline(always)]
-    pub fn fixed(preg: PReg) -> Self {
-        Operand::new(
-            VReg::invalid(),
-            OperandPolicy::FixedReg(preg),
-            OperandKind::Use,   // doesn't matter
-            OperandPos::Before, // doesn't matter
-        )
-    }
-
     #[inline(always)]
     pub fn vreg(self) -> VReg {
         let vreg_idx = ((self.bits as usize) & VReg::MAX) as usize;
@@ -401,14 +379,6 @@ impl Operand {
         }
     }
 
-    #[inline(always)]
-    pub fn as_fixed(self) -> Option<PReg> {
-        match (self.vreg().is_invalid(), self.policy()) {
-            (true, OperandPolicy::FixedReg(preg)) => Some(preg),
-            _ => None,
-        }
-    }
-
     #[inline(always)]
     pub fn bits(self) -> u32 {
         self.bits