[Go] 'defer' optimizing notes

Proposal:
https://go.googlesource.com/proposal/+/refs/heads/master/design/34481-opencoded-defers.md

Optimizing:

1. try not to run defer in a loop(for), which compiler will fallback to traditional defer chain implement
2. try to have less exit point inside a function, which compiler will fallback to traditional defer chain implement

Proposal by Example:

defer f1(a)
if cond {
 defer f2(b)
}
body...

compiles to:

deferBits |= 1<<0
tmpF1 = f1
tmpA = a
if cond {
 deferBits |= 1<<1
 tmpF2 = f2
 tmpB = b
}

body...

exit:
if deferBits & 1<<1 != 0 {
 deferBits &^= 1<<1
 tmpF2(tmpB)
}

if deferBits & 1<<0 != 0 {
 deferBits &^= 1<<0
 tmpF1(tmpA)
}