simple: flag superfluous boolean variable declarations in if statements

[NiloCK]

A potential new check:

Consider

if testValue := someFunc(); testValue {
  // do some stuff which doesn't use testValue
}

testValue passes the compiler's check for an unused variable, but is it really used? In my view, the code

if someFunc() {
  // do some stuff which doesn't use testValue
}

is unambiguously better.

[dgryski]

A quick note that if testValue is used in the block it can be statically replaced with a true (or false, depending).