cranelift: Add Bswap instruction (#1092) (#5147)

Adds Bswap to the Cranelift IR. Implements the Bswap instruction
in the x64 and aarch64 codegen backends. Cranelift users can now:
```
builder.ins().bswap(value)
```
to get a native byteswap instruction.

* x64: implements the 32- and 64-bit bswap instruction, following
the pattern set by similar unary instrutions (Neg and Not) - it
only operates on a dst register, but is parameterized with both
a src and dst which are expected to be the same register.

As x64 bswap instruction is only for 32- or 64-bit registers,
the 16-bit swap is implemented as a rotate left by 8.

Updated x64 RexFlags type to support emitting for single-operand
instructions like bswap

* aarch64: Bswap gets emitted as aarch64 rev16, rev32,
or rev64 instruction as appropriate.

* s390x: Bswap was already supported in backend, just had to add
a bit of plumbing

* For completeness, added bswap to the interpreter as well.

* added filetests and runtests for each ISA

* added bswap to fuzzgen, thanks to afonso360 for the code there

* 128-bit swaps are not yet implemented, that can be done later

cranelift/codegen/meta/src/shared/instructions.rs

@@ -643,6 +643,12 @@ pub(crate) fn define(
         TypeSetBuilder::new().ints(Interval::All).build(),
     );
 
+    let iSwappable = &TypeVar::new(
+        "iSwappable",
+        "A multi byte scalar integer type",
+        TypeSetBuilder::new().ints(16..128).build(),
+    );
+
     let iAddr = &TypeVar::new(
         "iAddr",
         "An integer address type",
@@ -2699,6 +2705,23 @@ pub(crate) fn define(
         .operands_out(vec![a]),
     );
 
+    let x = &Operand::new("x", iSwappable);
+    let a = &Operand::new("a", iSwappable);
+
+    ig.push(
+        Inst::new(
+            "bswap",
+            r#"
+        Reverse the byte order of an integer.
+
+        Reverses the bytes in ``x``.
+        "#,
+            &formats.unary,
+        )
+        .operands_in(vec![x])
+        .operands_out(vec![a]),
+    );
+
     let x = &Operand::new("x", Int);
     let a = &Operand::new("a", Int);