Implement single-use temporary folding optimization

[avrabe]

Feature Request

Fold single-use temporary variables back into their use sites to reduce local variable count.

Location

loom-core/src/lib.rs:7126

Current TODO

// TODO: Phase 4: Fold single-use temporaries

Example

Before:
local.tee $temp
...
local.get $temp  ;; only use

After:
local.tee $temp  ;; fold into use site

Impact

• Reduced local variable count
• Simpler IR for downstream passes

[avrabe]

Implemented single-use temp folding in commit 73a817d:

• Convert local.set $x; local.get $x -> local.tee $x
• More efficient encoding, same semantics
• Works together with dead store elimination for cascading optimizations