Support big- and little-endian lane order with bitcast (#5196)

Add a MemFlags operand to the bitcast instruction, where only the
`big` and `little` flags are accepted.  These define the lane order
to be used when casting between types of different lane counts.

Update all users to pass an appropriate MemFlags argument.

Implement lane swaps where necessary in the s390x back-end.

This is the final part necessary to fix
https://github.com/bytecodealliance/wasmtime/issues/4566.

cranelift/codegen/src/isa/riscv64/lower.isle

@@ -811,7 +811,7 @@
 )
 ;;;;;  Rules for `bitcast`;;;;;;;;;
 (rule
-   (lower (has_type out (bitcast v @ (value_type in_ty))))
+   (lower (has_type out (bitcast _ v @ (value_type in_ty))))
    (gen_moves v in_ty out))
 
 ;;;;;  Rules for `ceil`;;;;;;;;;