[lua] Replace IIFE discard with local _ assignment

[jdonaldson]

Summary

• Replace (function() return expr end)() with local _ = expr for discarded expression-statements in gen_iife_assign
• Affects TBinop(op != Assign) and TField at statement level in gen_block_element
• Simpler output, no closure overhead, avoids Lua's (-at-statement-start parsing ambiguity

Relates to #7350

Test plan

• All Lua unit tests pass (11,709 assertions)
• No (function() return patterns remain from gen_iife_assign
• 48 local _ = replacements in test output

[jdonaldson]

I'm trying to chip away at a few of the points where genlua creates a mess of expressions. One easy one is avoiding iifes for simple expressions that don't save return values.