Fix spillslot size bug in SIMD by removing type-dependent spillslot allocation.

This patch makes spillslot allocation, spilling and reloading all based
on register class only. Hence when we have a 32- or 64-bit value in a
128-bit XMM register on x86-64 or vector register on aarch64, this
results in larger spillslots and spills/restores.

Why make this change, if it results in less efficient stack-frame usage?
Simply put, it is safer: there is always a risk when allocating
spillslots or spilling/reloading that we get the wrong type and make the
spillslot or the store/load too small. This was one contributing factor
to CVE-2021-32629, and is now the source of a fuzzbug in SIMD code that
puns an arbitrary user-controlled vector constant over another
stackslot. (If this were a pointer, that could result in RCE. SIMD is
not yet on by default in a release, fortunately.

In particular, we have not been particularly careful about using moves
between values of different types, for example with `raw_bitcast` or
with certain SIMD operations, and such moves indicate to regalloc.rs
that vregs are in equivalence classes and some arbitrary vreg in the
class is provided when allocating the spillslot or spilling/reloading.
Since regalloc.rs does not track actual type, and since we haven't been
careful about moves, we can't really trust this "arbitrary vreg in
equivalence class" to provide accurate type information.

In the fix to CVE-2021-32629 we fixed this for integer registers by
always spilling/reloading 64 bits; this fix can be seen as the analogous
change for FP/vector regs.

cranelift/codegen/src/machinst/abi.rs

@@ -161,22 +161,13 @@ pub trait ABICallee {
     fn stack_args_size(&self) -> u32;
 
     /// Get the spill-slot size.
-    fn get_spillslot_size(&self, rc: RegClass, ty: Type) -> u32;
+    fn get_spillslot_size(&self, rc: RegClass) -> u32;
 
-    /// Generate a spill. The type, if known, is given; this can be used to
-    /// generate a store instruction optimized for the particular type rather
-    /// than the RegClass (e.g., only F64 that resides in a V128 register). If
-    /// no type is given, the implementation should spill the whole register.
-    fn gen_spill(&self, to_slot: SpillSlot, from_reg: RealReg, ty: Option<Type>) -> Self::I;
+    /// Generate a spill.
+    fn gen_spill(&self, to_slot: SpillSlot, from_reg: RealReg) -> Self::I;
 
-    /// Generate a reload (fill). As for spills, the type may be given to allow
-    /// a more optimized load instruction to be generated.
-    fn gen_reload(
-        &self,
-        to_reg: Writable<RealReg>,
-        from_slot: SpillSlot,
-        ty: Option<Type>,
-    ) -> Self::I;
+    /// Generate a reload (fill).
+    fn gen_reload(&self, to_reg: Writable<RealReg>, from_slot: SpillSlot) -> Self::I;
 }
 
 /// Trait implemented by an object that tracks ABI-related state and can