transform: various escape analysis improvements

[jakebailey]

Fixes #5378

This began as a fix to #5378, but like most of my PRs, ballooned into a bit more.

Each commit has a description, but the gist is that we are able to stack allocate more stuff that BigGo could also already do.

Unfortunately after #5220 the diff for the output is not helpful, so I have structured the PR such that all new tests are added in the first commit, that way looking at individual later commits show which allocs they remove.

[jakebailey]

Some raw data about the 787 std allocs this removes:

• stdalloc_removed.tsv
• stdalloc_removed_by_package.tsv

Representative removed variables/sites:

• runtime/hashmap.go:388 n := *m
• many new(big.Int) temporaries in crypto/dsa, crypto/elliptic, math/big
• fixed-size local arrays like tag, expectedTag, iv, mask, S in crypto code
• many local go/types temporaries in expression/type checking paths.

 archive/tar/common.go:426             blk        var blk block
 archive/zip/reader.go:639             File{}     if readDirectoryHeader(&File{}, rs) == nil
 archive/zip/reader_test.go:1082        f          var f File
 
 context/x_test.go:252                  check      check(o0, "o0", "", "", "")
 context/x_test.go:255                  check      check(o1, "o1", "c1k1", "", "")
 context/x_test.go:261                  check      check(o3, "o3", "", "c2k2", "c3k3")
 
 go/build/constraint/expr_test.go:76    p          p := &exprParser{s: tt.in}
 go/constant/value.go:216               mant       var mant big.Float
 go/constant/value.go:879               t          var t big.Float
 
 image/jpeg/dct_test.go:295-298         f          chained new(big.Float).Mul(...)
 image/jpeg/dct_test.go:352-356         f          chained new(big.Float).Mul/Quo(...)
 
 internal/chacha8rand/rand_test.go:17   s          var s State
 internal/chacha8rand/rand_test.go:68   blocks     var blocks [32]uint64
 internal/chacha8rand/rand_test.go:77   b1, b2     var b1, b2 [32]uint64
 
 slices/slices_test.go:1235             b          b = Insert(b, len(b)-1, 0)
 slices/slices_test.go:1253             b          b = Replace(b, len(b)-2, len(b)-1, 0, 0)
 slices/sort_test.go:237                fs         fs := []float64{...}
 
 time/sleep_test.go:647                 tr         var tr Timer
 time/tick_test.go:105                  tk         tk := &Ticker{C: c}
 time/tick_test.go:664                  tick       tick := &Ticker{C: c}
 
 tinygo/src/os/exec_linux_test.go:22    proc       StartProcess(..., &ProcAttr{})
 tinygo/src/runtime/hashmap.go:388      n          n := *m

[Copilot]

Pull request overview

This PR improves TinyGo’s heap-to-stack allocation optimization by enhancing escape analysis to recognize values that flow through returned parameters (including through aggregate operations), addressing unnecessary heap allocations like the one reported in #5378.

Changes:

• Extend transform/allocs.go escape analysis to track “returned” flows and follow them across calls (including through insertvalue / extractvalue) while guarding against recursion.
• Add new alloc-focused scenarios to transform/testdata/allocs2.go (returned pointer/slice cases, including a recursive case).
• Update golden outputs (allocs2.out.reason / allocs2.out.cover) to reflect the reduced/shifted allocation set after the analysis improvements.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
transform/allocs.go	Enhances escape analysis to better detect non-escaping returned-pointer flows and aggregate return propagation.
transform/testdata/allocs2.go	Adds new test cases covering returned pointer/slice scenarios and recursion behavior.
transform/testdata/allocs2.out.reason	Updates expected “reason” output for heap allocations after the analysis change.
transform/testdata/allocs2.out.cover	Updates expected coverage-style allocation output after the analysis change.

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