* In cases where we end up with empty branches like in
if a then jmp b else jmp b;
the flow can be replaced by a; jmp b.

The following functions is optimized as follows:
func f(a bool, x int) int {
        v := 0
        if a {
                v = -1
        } else {
                v = -1
        }
        return x | v
}

Before this change:
02819 (arith_ssa.go:362)  VARDEF "".~r2+16(FP)
02820 (arith_ssa.go:362)  MOVQ  $0, "".~r2+16(FP)
02821 (arith_ssa.go:362)  MOVB  "".a(FP), AX
02822 (arith_ssa.go:362)  TESTB AX, AX
02823 (arith_ssa.go:364)  JEQ 2824
02824 (arith_ssa.go:369)  VARDEF "".~r2+16(FP)
02825 (arith_ssa.go:369)  MOVQ  $-1, "".~r2+16(FP)
02826 (arith_ssa.go:369)  RET

After this change:
02819 (arith_ssa.go:362)  VARDEF "".~r2+16(FP)
02820 (arith_ssa.go:369)  VARDEF "".~r2+16(FP)
02821 (arith_ssa.go:369)  MOVQ  $-1, "".~r2+16(FP)
02822 (arith_ssa.go:369)  RET

Updates #14277

Change-Id: Ibe7d284f43406c704903632a4fcf2a4a64059686
Reviewed-on: https://go-review.googlesource.com/19464Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>