interp: fix phi instruction

I've discovered a bug in the implementation of the PHI instruction in
the interp package. This commit fixes the bug.

I've found this issue while investigating an issue with maps after
running interp per package.

interp/compiler.go

@@ -25,6 +25,7 @@ type function struct {
 // basicBlock represents a LLVM basic block and contains a slice of
 // instructions. The last instruction must be a terminator instruction.
 type basicBlock struct {
+	phiNodes     []instruction
 	instructions []instruction
 }
 
@@ -346,7 +347,13 @@ func (r *runner) compileFunction(llvmFn llvm.Value) *function {
 				// This error is handled when actually trying to interpret this
 				// instruction (to not trigger on code that won't be executed).
 			}
-			bb.instructions = append(bb.instructions, inst)
+			if inst.opcode == llvm.PHI {
+				// PHI nodes need to be treated specially, see the comment in
+				// interpreter.go for an explanation.
+				bb.phiNodes = append(bb.phiNodes, inst)
+			} else {
+				bb.instructions = append(bb.instructions, inst)
+			}
 		}
 	}
 	return fn