cmd/compile: improve codegen for nested if statements

[josharian]

As observed in CL 54651, these two semantically equivalent bits of code do not always compile equivalently:

if a {
  if b {
   // ...
  }
}

if a && b {
  // ...
}

This issue is to investigate why, and if the latter is systematically better, consider improving the former. Note that 'if a && b' gets special handling at Node -> SSA time; phiopt.go does some further related cleanup, but perhaps not enough.

cc @randall77

[mvdan]

Could this also apply to if a { if b := c(); b { ... } }?

[josharian]

@mvdan potentially, depending on the details of c() and how it relates to a.