[release-branch.go1.15] cmd/compile: do not assume TST and TEQ set V on arm
    
    These replacement rules assume that TST and TEQ set V. But TST and
    TEQ do not set V. This is a problem because instructions like LT are
    actually checking for N!=V. But with TST and TEQ not setting V, LT
    doesn't do anything meaningful. It's possible to construct trivial
    miscompilations from this, such as:
    
        package main
    
        var x = [4]int32{-0x7fffffff, 0x7fffffff, 2, 4}
    
        func main() {
            if x[0] > x[1] {
                panic("fail 1")
            }
            if x[2]&x[3] < 0 {
                panic("fail 2") // Fails here
            }
        }
    
    That first comparison sets V, via the CMP that subtracts the values
    causing the overflow. Then the second comparison operation thinks that
    it uses the result of TST, when it actually uses the V from CMP.
    
    Before this fix:
    
        TST             R0, R1
        BLT             loc_6C164
    
    After this fix:
    
        TST             R0, R1
        BMI             loc_6C164
    
    The BMI instruction checks the N flag, which TST sets.  This commit
    fixes the issue by using [LG][TE]noov instead of vanilla [LG][TE], and
    also adds a test case for the direct issue.
    
    Updates #42876.
    Fixes #42930.
    
    Change-Id: I13c62c88d18574247ad002b671b38d2d0b0fc6fa
    Reviewed-on: https://go-review.googlesource.com/c/go/+/282432
    Trust: Jason A. Donenfeld <Jason@zx2c4.com>
    Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
    TryBot-Result: Go Bot <gobot@golang.org>
    Reviewed-by: Cherry Zhang <cherryyz@google.com>