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/filetests/filetests/verifier/bitcast.clif

@@ -21,3 +21,34 @@ block0(v0: i64):
     return v1
 }
 
+; "little"/"big" flag modifier is ok
+function %bitcast_little(i32) -> f32 { ; Ok
+block0(v0: i32):
+    v1 = bitcast.f32 little v0
+    return v1
+}
+function %bitcast_big(i32) -> f32 { ; Ok
+block0(v0: i32):
+    v1 = bitcast.f32 big v0
+    return v1
+}
+
+; other flag modifiers are not ok
+function %bitcast_big(i32) -> f32 {
+block0(v0: i32):
+    v1 = bitcast.f32 notrap v0 ;  error: The bitcast instruction only accepts the `big` or `little` memory flags
+    return v1
+}
+function %bitcast_big(i32) -> f32 {
+block0(v0: i32):
+    v1 = bitcast.f32 aligned v0 ;  error: The bitcast instruction only accepts the `big` or `little` memory flags
+    return v1
+}
+
+; if lane counts differ, a byte order specifier is required
+function %bitcast_lanes(i32x4) -> i64x2 {
+block0(v0: i32x4):
+    v1 = bitcast.i64x2 v0 ;  error: Byte order specifier required for bitcast instruction changing lane count
+    return v1
+}
+