Update PHI node example in README to use more idiomatic structure

[Jumhyn]

When using LLVM's PHI nodes, creating a local variable to store the result in each preceding branch defeats the purpose of using PHI nodes in the first place (or, at the very least, obscures their purpose). With the use of local in the example originally, the program could be trivially rewritten as:

define double @calculateFibs(i1) {
entry:
  %local = alloca double
  %1 = icmp ne i1 %0, false
  br i1 %1, label %then, label %else

then:                                             ; preds = %entry
  store double 0x3F8702E05C0B8170, double* %local
  br label %merge

else:                                             ; preds = %entry
  store double 0x3F82C9FB4D812CA0, double* %local
  br label %merge

merge:                                            ; preds = %else, %then
  %retVal = load double, double* %local
  ret double %retVal
}

which sidesteps the need for a PHI node at all. I've rewritten the example to remove the unused local variable and to make the power of PHI nodes clearer.

[CodaFi]

Ah, true.

Could you update the comments that reference local too?

[CodaFi]

Better yet, do what I should have done here and store the PHI'd result into the local and return that. The goal isn't necessarily efficiency, but parity with the Swift example above this one.

[Jumhyn]

Updated. Kept the comments since now thenVal and elseVal really do represent the values being stored to local.

[CodaFi]

Thanks!

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