public RegisterSpec getAssignment(SsaInsn insn) {
    return insnAssignments.get(insn);
  }
Gets the named register being assigned by the given instruction, if previously stored in this
instance.

@param insn {@code non-null;} instruction in question
@return {@code null-ok;} the named register being assigned, if any
android/android_framework/dex-maker/src/main/java/com/android/dx/ssa/LocalVariableInfo.java