aarch64 isel: collect_address_addends: correctly handle ExtendOp::UXTW(negative immediate).

The current code doesn't correctly handle the case where `ExtendOp::UXTW` has
as source, a constant-producing insn that produces a negative (32-bit) value.
Then the value is incorrectly sign-extended to 64 bits (in fact, this has
already been done by `ctx.get_constant(insn)`), whereas it needs to be zero
extended.  The obvious fix, done here, is just to force bits 63:32 of the
extension to zero, hence zero-extending it.

cranelift/codegen/src/isa/aarch64/lower.rs

@@ -616,7 +616,7 @@ fn collect_address_addends<C: LowerCtx<I = Inst>>(
                         maybe_input_insn(ctx, extendee_input, Opcode::Iconst),
                         extendop,
                     ) {
-                        let value = ctx.get_constant(insn).unwrap() as i64;
+                        let value = (ctx.get_constant(insn).unwrap() & 0xFFFF_FFFF_u64) as i64;
                         offset += value;
                     } else {
                         let reg = put_input_in_reg(ctx, extendee_input, NarrowValueMode::None);