Fix Verilog codegen for narrow array update indices

[copybara-service]

Fix Verilog codegen for narrow array update indices

When codegen for Op::kArrayUpdate, target indexes were compared directly
against the 32-bit loop counter (index_expr == induction_var).

When index_expr was narrower than 32 bits, Verilog's context-determined
width rules expanded index_expr to 32 bits before applying the operation
creating the index. For example, if the index_expr is ~1. The 1 is expanded
to 32 bits first and then do ~ evaluated as 32'hFFFFFFFF (4294967295)
instead of 1'b1 zero-extended to 32'd1, causing the index comparison to
fail for all elements and reducing the array update to a no-op in generated
Verilog.

This change explicitly zero-extends narrow index expressions to 32 bits via
concatenation ({ (32 - width)'b0, index_expr }), creating a self-determined
context so unary operations evaluate at their native width before
comparison.