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/src/isa/x64/lower.isle

@@ -2065,6 +2065,19 @@
                             hi32)))
         swap32))
 
+;; Rules for `bswap` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+;; x64 bswap instruction is only for 32- or 64-bit swaps
+;; implement the 16-bit swap as a rotl by 8
+(rule (lower (has_type $I16 (bswap src)))
+      (x64_rotl $I16 src (Imm8Reg.Imm8 8)))
+
+(rule (lower (has_type $I32 (bswap src)))
+      (x64_bswap $I32 src))
+
+(rule (lower (has_type $I64 (bswap src)))
+      (x64_bswap $I64 src))
+
 ;; Rules for `is_null` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 ;; Null references are represented by the constant value `0`.