[release-branch.go1.15] cmd/compile: fix escape analysis of heap-allocated results
    
    One of escape analysis's responsibilities is to summarize whether/how
    each function parameter flows to the heap so we can correctly
    incorporate those flows into callers' escape analysis data flow
    graphs.
    
    As an optimization, we separately record when parameters flow to
    result parameters, so that we can more precisely analyze parameter
    flows based on how the results are used at the call site. However, if
    a named result parameter itself needs to be heap allocated, this
    optimization isn't safe and the parameter needs to be recorded as
    flowing to heap rather than flowing to result.
    
    Escape analysis used to get this correct because it conservatively
    rewalked the data-flow graph multiple times. So even though it would
    incorrectly record the result parameter flow, it would separately find
    a flow to the heap. However, CL 196811 (specifically, case 3)
    optimized the walking logic to reduce unnecessary rewalks causing us
    to stop finding the extra heap flow.
    
    This CL fixes the issue by correcting location.leakTo to be sensitive
    to sink.escapes and not record result-flows when the result parameter
    escapes to the heap.
    
    Fixes #44658.
    
    Change-Id: I48742ed35a6cab591094e2d23a439e205bd65c50
    Reviewed-on: https://go-review.googlesource.com/c/go/+/297289
    Trust: Matthew Dempsky <mdempsky@google.com>
    Run-TryBot: Matthew Dempsky <mdempsky@google.com>
    TryBot-Result: Go Bot <gobot@golang.org>
    Reviewed-by: Keith Randall <khr@golang.org>
    Reviewed-on: https://go-review.googlesource.com/c/go/+/297291